669. Trim a Binary Search Tree

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:

tree

L = 1
R = 2

Output:

trimed

Example 2:

Input:

tree

L = 1
R = 3

Output:

trimed

Solution

DFS

/**
 * 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 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;
        }
    }
}