BalancedBinaryTree.java

/*
 Author:     King, higuige@gmail.com
 Date:       Oct 07, 2014
 Problem:    Balanced Binary Tree
 Difficulty: Easy
 Source:     https://oj.leetcode.com/problems/balanced-binary-tree/
 Notes:
 Given a binary tree, determine if it is height-balanced.
 For this problem, a height-balanced binary tree is defined as a binary tree in which 
 the depth of the two subtrees of every node never differ by more than 1.

 Solution: DFS.
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        return isBalancedRe(root) > -1;
    }
    public int isBalancedRe(TreeNode root) {
        if (root == null) return 0;
        int left = isBalancedRe(root.left);
        int right = isBalancedRe(root.right);
        if (left == -1 || right == -1) return -1;
        if (Math.abs(left - right) > 1) return -1;
        return Math.max(left, right) + 1;
    }
}