[Leetcode] 669. Trim a Binary Search Tree 解题报告_java:as we know,the shape of a binary search tree -优快云博客

本文介绍了一种算法，用于修剪二叉搜索树，使其所有元素的值位于指定的最低值L和最高值R之间。通过递归地修剪不符合条件的节点，确保最终树中所有节点的值都在给定范围内。

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题目：

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

思路：

判断根节点的值和[L,R]的关系：如果比L还小，根节点也要被trim，所以返回对左子树的调用即可；如果比R还大，根节点同样要被trim，所以返回对右子树的调用即可。否则根节点就需要保留，所以我们分别trim其左右子树，然后返回根节点。

代码：

/**
 * 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 == NULL) {
            return NULL;
        }
        if (root->val < L) {
            return trimBST(root->right, L, R);
        }
        else if (root->val > R) {
            return trimBST(root->left, L, R);
        }
        else {
            root->left = trimBST(root->left, L, R);
            root->right = trimBST(root->right, L, R);
            return root;
        }
    }
};