B+ 树

引入

B+ 树是 B 树的一个升级，它比 B 树更适合实际应用中操作系统的文件索引和数据库索引。目前现代关系型数据库最广泛的支持索引结构就是 B+ 树。

B+ 树是一种多叉排序树，即每个节点通常有多个孩子。一棵 B+ 树包含根节点、内部节点和叶子节点。根节点可能是一个叶子节点，也可能是一个包含两个或两个以上孩子节点的节点。

B+ 树的特点是能够保持数据稳定有序，其插入与修改拥有较稳定的对数时间复杂度。B+ 树元素自底向上插入，这与二叉树恰好相反。

首先介绍一棵阶 B+ 树的特性。表示这个树的每一个节点最多可以拥有的子节点个数。一棵阶的 B+ 树和 B 树的差异在于：

有棵子树的节点中含有个关键字（即将区间分为个子区间，每个子区间对应一棵子树）。
所有叶子节点中包含了全部关键字的信息，及指向含这些关键字记录的指针，且叶子节点本身依关键字的大小自小而大顺序链接。
所有的非叶子节点可以看成是索引部分，节点中仅含有其子树（根节点）中的最大（或最小）关键字。
除根节点外，其他所有节点中所含关键字的个数最少有 $\lceil \dfrac{m}{2} \rceil$ （注意：B 树中除根以外的所有非叶子节点至少有 $\lceil \dfrac{m}{2} \rceil$ 棵子树）。

同时，B+ 树为了方便范围查询，叶子节点之间还用指针串联起来。

以下是一棵 B+ 树的典型结构：

B+ 树相比于 B 树的优势

由于索引节点上只有索引而没有数据，所以索引节点上能存储比 B 树更多的索引，这样树的高度就会更矮。树的高度越矮，磁盘寻道的次数就会越少。

因为数据都集中在叶子节点，而所有叶子节点的高度相同，那么可以在叶子节点中增加前后指针，指向同一个父节点的相邻兄弟节点，这样可以更好地支持查询一个值的前驱或后继，使连续访问更容易实现。

比如这样的 SQL 语句：select * from tbl where t > 10，如果使用 B+ 树存储数据的话，可以首先定位到数据为 10 的节点，再沿着它的 next 指针一路找到所有在该叶子节点右边的叶子节点，返回这些节点包含的数据。

而如果使用 B 树结构，由于数据既可以存储在内部节点也可以存储在叶子节点，连续访问的实现会更加繁琐（需要在树的内部结构中进行移动）。

过程

与 B 树类似，B+ 树的基本操作有查找，遍历，插入，删除。

查找

B+ 树的查找过程和 B 树类似。假设需要查找的键值是，那么从根节点开始，从上到下递归地遍历树。在每一层上，搜索的范围被减小到包含搜索值的子树中。

一个实例：在如下这棵 B+ 树上查找 45。

先和根节点比较

因为根节点的键值比 45 要小，所以去往根节点的右子树查找

因为 45 比 35 大，所以要与右边的索引相比

右侧的索引也为 45，所以要去往该节点的右子树继续查找

然后就可以找到 45

需要注意的是，在查找时，若非叶子节点上的关键字等于给定值，并不终止，而是继续向下直到叶子节点。因此，在 B+ 树中，不管查找成功与否，每次查找都是走了一条从根到叶子节点的路径。其余同 B 树的查找类似。

查找一个键的代码如下：

实现

T find(V key) {
  int i = 0;
  while (i < this.number) {
    if (key.compareTo((V)this.keys[i]) <= 0) break;
    i++;
  }
  if (this.number == i) return null;
  return this.childs[i].find(key);
}

遍历

B+ 树只在叶子节点的层级上就可以实现整棵树的遍历。从根节点出发一路搜索到最左端的叶子节点之后即可根据指针遍历。

插入

B+ 树的插入算法与 B 树的相近：

若为空树，创建一个叶子节点，然后将记录插入其中，此时这个叶子节点也是根节点，插入操作结束。
针对叶子类型节点：根据关键字找到叶子节点，向这个叶子节点插入记录。插入后，若当前节点关键字的个数小于，则插入结束。否则将这个叶子节点分裂成左右两个叶子节点，左叶子节点包含前个记录，右节点包含剩下的记录，将第个记录的关键字进位到父节点中（父节点一定是索引类型节点），进位到父节点的关键字左孩子指针向左节点，右孩子指针向右节点。将当前节点的指针指向父节点，然后执行第 3 步。
针对索引类型节点（内部节点）：若当前节点关键字的个数小于等于，则插入结束。否则，将这个索引类型节点分裂成两个索引节点，左索引节点包含前个 key，右节点包含个 key，将第个关键字进位到父节点中，进位到父节点的关键字左孩子指向左节点，进位到父节点的关键字右孩子指向右节点。将当前节点的指针指向父节点，然后重复这一步。

比如在下图的 B+ 树中，插入新的数据 10：

由于插入节点在插入之后并没有溢出，所以可以直接变成。

而如下图的 B+ 树中，插入数据 4：

由于所在节点在插入之后数据溢出，因此需要分裂为两个新的节点，同时调整父节点的索引数据：

分裂成了和，因此需要在这两个节点之间新增一个索引值，这个值应该满足：

大于左子树的最大值；
小于等于右子树的最小值。

综上，需要在父节点中新增索引 4 和两个指向新节点的指针。

更多的例子可以参考演示网站 BPlustree

插入一个键的代码如下：

实现

void BPTree::insert(int x) {
  if (root == NULL) {
    root = new Node;
    root->key[0] = x;
    root->IS_LEAF = true;
    root->size = 1;
    root->parent = NULL;
  } else {
    Node* cursor = root;
    Node* parent;

    while (cursor->IS_LEAF == false) {
      parent = cursor;
      for (int i = 0; i < cursor->size; i++) {
        if (x < cursor->key[i]) {
          cursor = cursor->ptr[i];
          break;
        }

        if (i == cursor->size - 1) {
          cursor = cursor->ptr[i + 1];
          break;
        }
      }
    }
    if (cursor->size < MAX) {
      insertVal(x, cursor);
      cursor->parent = parent;
      cursor->ptr[cursor->size] = cursor->ptr[cursor->size - 1];
      cursor->ptr[cursor->size - 1] = NULL;
    } else
      split(x, parent, cursor);
  }
}

void BPTree::split(int x, Node* parent, Node* cursor) {
  Node* LLeaf = new Node;
  Node* RLeaf = new Node;
  insertVal(x, cursor);
  LLeaf->IS_LEAF = RLeaf->IS_LEAF = true;
  LLeaf->size = (MAX + 1) / 2;
  RLeaf->size = (MAX + 1) - (MAX + 1) / 2;
  for (int i = 0; i < MAX + 1; i++) LLeaf->ptr[i] = cursor->ptr[i];
  LLeaf->ptr[LLeaf->size] = RLeaf;
  RLeaf->ptr[RLeaf->size] = LLeaf->ptr[MAX];
  LLeaf->ptr[MAX] = NULL;
  for (int i = 0; i < LLeaf->size; i++) {
    LLeaf->key[i] = cursor->key[i];
  }
  for (int i = 0, j = LLeaf->size; i < RLeaf->size; i++, j++) {
    RLeaf->key[i] = cursor->key[j];
  }
  if (cursor == root) {
    Node* newRoot = new Node;
    newRoot->key[0] = RLeaf->key[0];
    newRoot->ptr[0] = LLeaf;
    newRoot->ptr[1] = RLeaf;
    newRoot->IS_LEAF = false;
    newRoot->size = 1;
    root = newRoot;
    LLeaf->parent = RLeaf->parent = newRoot;
  } else {
    insertInternal(RLeaf->key[0], parent, LLeaf, RLeaf);
  }
}

void BPTree::insertInternal(int x, Node* cursor, Node* LLeaf, Node* RRLeaf) {
  if (cursor->size < MAX) {
    auto i = insertVal(x, cursor);
    for (int j = cursor->size; j > i + 1; j--) {
      cursor->ptr[j] = cursor->ptr[j - 1];
    }
    cursor->ptr[i] = LLeaf;
    cursor->ptr[i + 1] = RRLeaf;
  }

  else {
    Node* newLchild = new Node;
    Node* newRchild = new Node;
    Node* virtualPtr[MAX + 2];
    for (int i = 0; i < MAX + 1; i++) {
      virtualPtr[i] = cursor->ptr[i];
    }
    int i = insertVal(x, cursor);
    for (int j = MAX + 2; j > i + 1; j--) {
      virtualPtr[j] = virtualPtr[j - 1];
    }
    virtualPtr[i] = LLeaf;
    virtualPtr[i + 1] = RRLeaf;
    newLchild->IS_LEAF = newRchild->IS_LEAF = false;
    // 这里和叶子节点有区别
    newLchild->size = (MAX + 1) / 2;
    newRchild->size = MAX - (MAX + 1) / 2;
    for (int i = 0; i < newLchild->size; i++) {
      newLchild->key[i] = cursor->key[i];
    }
    for (int i = 0, j = newLchild->size + 1; i < newRchild->size; i++, j++) {
      newRchild->key[i] = cursor->key[j];
    }
    for (int i = 0; i < LLeaf->size + 1; i++) {
      newLchild->ptr[i] = virtualPtr[i];
    }
    for (int i = 0, j = LLeaf->size + 1; i < RRLeaf->size + 1; i++, j++) {
      newRchild->ptr[i] = virtualPtr[j];
    }
    if (cursor == root) {
      Node* newRoot = new Node;
      newRoot->key[0] = cursor->key[newLchild->size];
      newRoot->ptr[0] = newLchild;
      newRoot->ptr[1] = newRchild;
      newRoot->IS_LEAF = false;
      newRoot->size = 1;
      root = newRoot;
      newLchild->parent = newRchild->parent = newRoot;
    } else {
      insertInternal(cursor->key[newLchild->size], cursor->parent, newLchild,
                     newRchild);
    }
  }
}

删除

B+ 树的删除也仅在叶子节点中进行，当叶子节点中的最大关键字被删除时，其在非叶子节点中的值可以作为一个分界关键字存在。若因删除而使节点中关键字的个数少于 $\lceil \dfrac{m}{2} \rceil$ 时，其和兄弟节点的合并过程亦和 B 树类似。

具体步骤如下：

首先查询到键值所在的叶子节点，删除该叶子节点的数据。
如果删除叶子节点之后的数据数量，满足 B+ 树的平衡条件，则直接返回。
否则，就需要做平衡操作：如果该叶子节点的左右兄弟节点的数据量可以借用，就借用过来满足平衡条件。否则，就与相邻的兄弟节点合并成一个新的子节点了。

在上面平衡操作中，如果是进行了合并操作，就需要向上修正父节点的指针：删除被合并节点的键值以及指针。

由于做了删除操作，可能父节点也会不平衡，那么就按照前面的步骤也对父节点进行重新平衡操作，这样一直到某个节点平衡为止。

可以参考 B 树中的删除章节。

实现

// Deletion operation on a B+ tree in C++
#include <climits>
#include <fstream>
#include <iostream>
#include <sstream>
using namespace std;
int MAX = 3;

class BPTree;

class Node {
  bool IS_LEAF;
  int *key, size;
  Node **ptr;
  friend class BPTree;

 public:
  Node();
};

class BPTree {
  Node *root;
  void insertInternal(int, Node *, Node *);
  void removeInternal(int, Node *, Node *);
  Node *findParent(Node *, Node *);

 public:
  BPTree();
  void search(int);
  void insert(int);
  void remove(int);
  void display(Node *);
  Node *getRoot();
};

Node::Node() {
  key = new int[MAX];
  ptr = new Node *[MAX + 1];
}

BPTree::BPTree() { root = NULL; }

void BPTree::insert(int x) {
  if (root == NULL) {
    root = new Node;
    root->key[0] = x;
    root->IS_LEAF = true;
    root->size = 1;
  } else {
    Node *cursor = root;
    Node *parent;
    while (cursor->IS_LEAF == false) {
      parent = cursor;
      for (int i = 0; i < cursor->size; i++) {
        if (x < cursor->key[i]) {
          cursor = cursor->ptr[i];
          break;
        }
        if (i == cursor->size - 1) {
          cursor = cursor->ptr[i + 1];
          break;
        }
      }
    }
    if (cursor->size < MAX) {
      int i = 0;
      while (x > cursor->key[i] && i < cursor->size) i++;
      for (int j = cursor->size; j > i; j--) {
        cursor->key[j] = cursor->key[j - 1];
      }
      cursor->key[i] = x;
      cursor->size++;
      cursor->ptr[cursor->size] = cursor->ptr[cursor->size - 1];
      cursor->ptr[cursor->size - 1] = NULL;
    } else {
      Node *newLeaf = new Node;
      int virtualNode[MAX + 1];
      for (int i = 0; i < MAX; i++) {
        virtualNode[i] = cursor->key[i];
      }
      int i = 0, j;
      while (x > virtualNode[i] && i < MAX) i++;
      for (int j = MAX + 1; j > i; j--) {
        virtualNode[j] = virtualNode[j - 1];
      }
      virtualNode[i] = x;
      newLeaf->IS_LEAF = true;
      cursor->size = (MAX + 1) / 2;
      newLeaf->size = MAX + 1 - (MAX + 1) / 2;
      cursor->ptr[cursor->size] = newLeaf;
      newLeaf->ptr[newLeaf->size] = cursor->ptr[MAX];
      cursor->ptr[MAX] = NULL;
      for (i = 0; i < cursor->size; i++) {
        cursor->key[i] = virtualNode[i];
      }
      for (i = 0, j = cursor->size; i < newLeaf->size; i++, j++) {
        newLeaf->key[i] = virtualNode[j];
      }
      if (cursor == root) {
        Node *newRoot = new Node;
        newRoot->key[0] = newLeaf->key[0];
        newRoot->ptr[0] = cursor;
        newRoot->ptr[1] = newLeaf;
        newRoot->IS_LEAF = false;
        newRoot->size = 1;
        root = newRoot;
      } else {
        insertInternal(newLeaf->key[0], parent, newLeaf);
      }
    }
  }
}

void BPTree::insertInternal(int x, Node *cursor, Node *child) {
  if (cursor->size < MAX) {
    int i = 0;
    while (x > cursor->key[i] && i < cursor->size) i++;
    for (int j = cursor->size; j > i; j--) {
      cursor->key[j] = cursor->key[j - 1];
    }
    for (int j = cursor->size + 1; j > i + 1; j--) {
      cursor->ptr[j] = cursor->ptr[j - 1];
    }
    cursor->key[i] = x;
    cursor->size++;
    cursor->ptr[i + 1] = child;
  } else {
    Node *newInternal = new Node;
    int virtualKey[MAX + 1];
    Node *virtualPtr[MAX + 2];
    for (int i = 0; i < MAX; i++) {
      virtualKey[i] = cursor->key[i];
    }
    for (int i = 0; i < MAX + 1; i++) {
      virtualPtr[i] = cursor->ptr[i];
    }
    int i = 0, j;
    while (x > virtualKey[i] && i < MAX) i++;
    for (int j = MAX + 1; j > i; j--) {
      virtualKey[j] = virtualKey[j - 1];
    }
    virtualKey[i] = x;
    for (int j = MAX + 2; j > i + 1; j--) {
      virtualPtr[j] = virtualPtr[j - 1];
    }
    virtualPtr[i + 1] = child;
    newInternal->IS_LEAF = false;
    cursor->size = (MAX + 1) / 2;
    newInternal->size = MAX - (MAX + 1) / 2;
    for (i = 0, j = cursor->size + 1; i < newInternal->size; i++, j++) {
      newInternal->key[i] = virtualKey[j];
    }
    for (i = 0, j = cursor->size + 1; i < newInternal->size + 1; i++, j++) {
      newInternal->ptr[i] = virtualPtr[j];
    }
    if (cursor == root) {
      Node *newRoot = new Node;
      newRoot->key[0] = cursor->key[cursor->size];
      newRoot->ptr[0] = cursor;
      newRoot->ptr[1] = newInternal;
      newRoot->IS_LEAF = false;
      newRoot->size = 1;
      root = newRoot;
    } else {
      insertInternal(cursor->key[cursor->size], findParent(root, cursor),
                     newInternal);
    }
  }
}

Node *BPTree::findParent(Node *cursor, Node *child) {
  Node *parent;
  if (cursor->IS_LEAF || (cursor->ptr[0])->IS_LEAF) {
    return NULL;
  }
  for (int i = 0; i < cursor->size + 1; i++) {
    if (cursor->ptr[i] == child) {
      parent = cursor;
      return parent;
    } else {
      parent = findParent(cursor->ptr[i], child);
      if (parent != NULL) return parent;
    }
  }
  return parent;
}

void BPTree::remove(int x) {
  if (root == NULL) {
    cout << "Tree empty\n";
  } else {
    Node *cursor = root;
    Node *parent;
    int leftSibling, rightSibling;
    while (cursor->IS_LEAF == false) {
      for (int i = 0; i < cursor->size; i++) {
        parent = cursor;
        leftSibling = i - 1;
        rightSibling = i + 1;
        if (x < cursor->key[i]) {
          cursor = cursor->ptr[i];
          break;
        }
        if (i == cursor->size - 1) {
          leftSibling = i;
          rightSibling = i + 2;
          cursor = cursor->ptr[i + 1];
          break;
        }
      }
    }
    bool found = false;
    int pos;
    for (pos = 0; pos < cursor->size; pos++) {
      if (cursor->key[pos] == x) {
        found = true;
        break;
      }
    }
    if (!found) {
      cout << "Not found\n";
      return;
    }
    for (int i = pos; i < cursor->size; i++) {
      cursor->key[i] = cursor->key[i + 1];
    }
    cursor->size--;
    if (cursor == root) {
      for (int i = 0; i < MAX + 1; i++) {
        cursor->ptr[i] = NULL;
      }
      if (cursor->size == 0) {
        cout << "Tree died\n";
        delete[] cursor->key;
        delete[] cursor->ptr;
        delete cursor;
        root = NULL;
      }
      return;
    }
    cursor->ptr[cursor->size] = cursor->ptr[cursor->size + 1];
    cursor->ptr[cursor->size + 1] = NULL;
    if (cursor->size >= (MAX + 1) / 2) {
      return;
    }
    if (leftSibling >= 0) {
      Node *leftNode = parent->ptr[leftSibling];
      if (leftNode->size >= (MAX + 1) / 2 + 1) {
        for (int i = cursor->size; i > 0; i--) {
          cursor->key[i] = cursor->key[i - 1];
        }
        cursor->size++;
        cursor->ptr[cursor->size] = cursor->ptr[cursor->size - 1];
        cursor->ptr[cursor->size - 1] = NULL;
        cursor->key[0] = leftNode->key[leftNode->size - 1];
        leftNode->size--;
        leftNode->ptr[leftNode->size] = cursor;
        leftNode->ptr[leftNode->size + 1] = NULL;
        parent->key[leftSibling] = cursor->key[0];
        return;
      }
    }
    if (rightSibling <= parent->size) {
      Node *rightNode = parent->ptr[rightSibling];
      if (rightNode->size >= (MAX + 1) / 2 + 1) {
        cursor->size++;
        cursor->ptr[cursor->size] = cursor->ptr[cursor->size - 1];
        cursor->ptr[cursor->size - 1] = NULL;
        cursor->key[cursor->size - 1] = rightNode->key[0];
        rightNode->size--;
        rightNode->ptr[rightNode->size] = rightNode->ptr[rightNode->size + 1];
        rightNode->ptr[rightNode->size + 1] = NULL;
        for (int i = 0; i < rightNode->size; i++) {
          rightNode->key[i] = rightNode->key[i + 1];
        }
        parent->key[rightSibling - 1] = rightNode->key[0];
        return;
      }
    }
    if (leftSibling >= 0) {
      Node *leftNode = parent->ptr[leftSibling];
      for (int i = leftNode->size, j = 0; j < cursor->size; i++, j++) {
        leftNode->key[i] = cursor->key[j];
      }
      leftNode->ptr[leftNode->size] = NULL;
      leftNode->size += cursor->size;
      leftNode->ptr[leftNode->size] = cursor->ptr[cursor->size];
      removeInternal(parent->key[leftSibling], parent, cursor);
      delete[] cursor->key;
      delete[] cursor->ptr;
      delete cursor;
    } else if (rightSibling <= parent->size) {
      Node *rightNode = parent->ptr[rightSibling];
      for (int i = cursor->size, j = 0; j < rightNode->size; i++, j++) {
        cursor->key[i] = rightNode->key[j];
      }
      cursor->ptr[cursor->size] = NULL;
      cursor->size += rightNode->size;
      cursor->ptr[cursor->size] = rightNode->ptr[rightNode->size];
      cout << "Merging two leaf nodes\n";
      removeInternal(parent->key[rightSibling - 1], parent, rightNode);
      delete[] rightNode->key;
      delete[] rightNode->ptr;
      delete rightNode;
    }
  }
}

void BPTree::removeInternal(int x, Node *cursor, Node *child) {
  if (cursor == root) {
    if (cursor->size == 1) {
      if (cursor->ptr[1] == child) {
        delete[] child->key;
        delete[] child->ptr;
        delete child;
        root = cursor->ptr[0];
        delete[] cursor->key;
        delete[] cursor->ptr;
        delete cursor;
        cout << "Changed root node\n";
        return;
      } else if (cursor->ptr[0] == child) {
        delete[] child->key;
        delete[] child->ptr;
        delete child;
        root = cursor->ptr[1];
        delete[] cursor->key;
        delete[] cursor->ptr;
        delete cursor;
        cout << "Changed root node\n";
        return;
      }
    }
  }
  int pos;
  for (pos = 0; pos < cursor->size; pos++) {
    if (cursor->key[pos] == x) {
      break;
    }
  }
  for (int i = pos; i < cursor->size; i++) {
    cursor->key[i] = cursor->key[i + 1];
  }
  for (pos = 0; pos < cursor->size + 1; pos++) {
    if (cursor->ptr[pos] == child) {
      break;
    }
  }
  for (int i = pos; i < cursor->size + 1; i++) {
    cursor->ptr[i] = cursor->ptr[i + 1];
  }
  cursor->size--;
  if (cursor->size >= (MAX + 1) / 2 - 1) {
    return;
  }
  if (cursor == root) return;
  Node *parent = findParent(root, cursor);
  int leftSibling, rightSibling;
  for (pos = 0; pos < parent->size + 1; pos++) {
    if (parent->ptr[pos] == cursor) {
      leftSibling = pos - 1;
      rightSibling = pos + 1;
      break;
    }
  }
  if (leftSibling >= 0) {
    Node *leftNode = parent->ptr[leftSibling];
    if (leftNode->size >= (MAX + 1) / 2) {
      for (int i = cursor->size; i > 0; i--) {
        cursor->key[i] = cursor->key[i - 1];
      }
      cursor->key[0] = parent->key[leftSibling];
      parent->key[leftSibling] = leftNode->key[leftNode->size - 1];
      for (int i = cursor->size + 1; i > 0; i--) {
        cursor->ptr[i] = cursor->ptr[i - 1];
      }
      cursor->ptr[0] = leftNode->ptr[leftNode->size];
      cursor->size++;
      leftNode->size--;
      return;
    }
  }
  if (rightSibling <= parent->size) {
    Node *rightNode = parent->ptr[rightSibling];
    if (rightNode->size >= (MAX + 1) / 2) {
      cursor->key[cursor->size] = parent->key[pos];
      parent->key[pos] = rightNode->key[0];
      for (int i = 0; i < rightNode->size - 1; i++) {
        rightNode->key[i] = rightNode->key[i + 1];
      }
      cursor->ptr[cursor->size + 1] = rightNode->ptr[0];
      for (int i = 0; i < rightNode->size; ++i) {
        rightNode->ptr[i] = rightNode->ptr[i + 1];
      }
      cursor->size++;
      rightNode->size--;
      return;
    }
  }
  if (leftSibling >= 0) {
    Node *leftNode = parent->ptr[leftSibling];
    leftNode->key[leftNode->size] = parent->key[leftSibling];
    for (int i = leftNode->size + 1, j = 0; j < cursor->size; j++) {
      leftNode->key[i] = cursor->key[j];
    }
    for (int i = leftNode->size + 1, j = 0; j < cursor->size + 1; j++) {
      leftNode->ptr[i] = cursor->ptr[j];
      cursor->ptr[j] = NULL;
    }
    leftNode->size += cursor->size + 1;
    cursor->size = 0;
    removeInternal(parent->key[leftSibling], parent, cursor);
  } else if (rightSibling <= parent->size) {
    Node *rightNode = parent->ptr[rightSibling];
    cursor->key[cursor->size] = parent->key[rightSibling - 1];
    for (int i = cursor->size + 1, j = 0; j < rightNode->size; j++) {
      cursor->key[i] = rightNode->key[j];
    }
    for (int i = cursor->size + 1, j = 0; j < rightNode->size + 1; j++) {
      cursor->ptr[i] = rightNode->ptr[j];
      rightNode->ptr[j] = NULL;
    }
    cursor->size += rightNode->size + 1;
    rightNode->size = 0;
    removeInternal(parent->key[rightSibling - 1], parent, rightNode);
  }
}

void BPTree::display(Node *cursor) {
  if (cursor != NULL) {
    for (int i = 0; i < cursor->size; i++) {
      cout << cursor->key[i] << " ";
    }
    cout << "\n";
    if (cursor->IS_LEAF != true) {
      for (int i = 0; i < cursor->size + 1; i++) {
        display(cursor->ptr[i]);
      }
    }
  }
}

Node *BPTree::getRoot() { return root; }

int main() {
  BPTree node;
  node.insert(5);
  node.insert(15);
  node.insert(25);
  node.insert(35);
  node.insert(45);

  node.display(node.getRoot());

  node.remove(15);

  node.display(node.getRoot());
}

参考资料

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