leetcode讲解--669. Trim a Binary Search Tree

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

题目地址

讲解

这道题对锻炼递归能力很有帮助。思路如下：如果当前节点大于R或小于L，肯定是要抛弃的。如果大于R，就返回其左子树（要对左子树继续裁剪），如果小于L，就返回其右子树（要对右子树继续裁剪）。如果在L和R之间，就继续遍历其左右进行裁剪。

Java代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if(root==null){
            return root;
        }
        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;
    }
}