669-修剪二叉搜索树

LeetCode 669. Trim a Binary Search Tree

LeetCode-669

Given the root of a binary search tree and the lowest and highest boundaries as low and high, trim the tree so that all its elements lies in [low, high]. Trimming the tree should not change the relative structure of the elements that will remain in the tree (i.e., any node’s descendant should remain a descendant). It can be proven that there is a unique answer.

Return the root of the trimmed binary search tree. Note that the root may change depending on the given bounds.

Example 1:

Input: root = [1,0,2], low = 1, high = 2
Output: [1,null,2]

Example 2:

Input: root = [3,0,4,null,2,null,null,1], low = 1, high = 3
Output: [3,2,null,1]

method

• 当前节点是要删除的

  • 如果节点值小于左区间，返回递归处理的右子树，因为左子树肯定也都小于，直接扔掉
  • 如果节点值大于右区间，返回递归处理的左子树，扔掉右子树

• 当前节点保留，分别递归处理左子树和右子树

TreeNode* trimBST(TreeNode* root, int low, int high) {
    if (!root) return root;
    if (root->val < low) return trimBST(root->right, low, high);    // 直接返回右子树，相当于把当前节点删掉了
    if (root->val > high) return trimBST(root->left, low, high);
    root->left = trimBST(root->left, low, high);    // 递归左子树
    root->right = trimBST(root->right, low, high);  // 递归右子树
    return root;
}