LeetCode-Trim a Binary Search Tree

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Description：
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

题意：要求对一颗BST树进行剪枝操作，使得所有节点的值都在[L, R](L <= R)范围内；返回剪枝后的BST树；

解法：对于一颗BST树有如下三个特点

• root.val > root.left.val
• root.val < root.right.val
• 当我们对BST树利用中序遍历时可以得到一个升序排序的序列；

利用这三个特点，我们对每个节点root，进行如下判断

• if root.val < L：则满足条件的节点必然在root.right部分
• if root.val > R: 则满足条件的节点必然在root.left部分
• 否则，root是我们所要找的节点

Java

/**
 * 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);
        }
        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;
    }
}