669. Trim a Binary Search Tree*

669. Trim a Binary Search Tree*

https://leetcode.com/problems/trim-a-binary-search-tree/

题目描述

Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example 1:

Input: 
    1
   / \
  0   2

  L = 1
  R = 2

Output: 
    1
      \
       2

Example 2:

Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

C++ 实现 1

可以理解为按照后序遍历来处理, 先 trim 左子树, 再 trim 右子树, 最后处理根节点. 对于二分搜索树, 其特点是左子树小于根节点, 右子树大于根节点, 所以对于 root->val < L 以及 root->val > R 两种特殊情况需要专门进行处理.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int L, int R) {
        if (!root) return nullptr;
        if (root->val < L) return trimBST(root->right, L, R);
        else if (root->val > R) return trimBST(root->left, L, R);
        root->left = trimBST(root->left, L, R);
        root->right = trimBST(root->right, L, R);
        return root;
    }
};