Balanced Binary Tree.java

/*
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.

Example
Given binary tree A = {3,9,20,#,#,15,7}, B = {3,#,20,15,7}

A)  3            B)    3 
   / \                  \
  9  20                 20
    /  \                / \
   15   7              15  7
   
time = O(nlogn) -> O(n)
space = O(n)
*/

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    public boolean isBalanced(TreeNode root) {
        // write your code here
        if (root == null) {
            return true;
        }
        return getHeight(root) != -1;
    }
    
    private int getHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = getHeight(root.left);
        if (left == -1) {
            return -1;
        }
        int right = getHeight(root.right);
        if (right == -1) {
            return -1;
        }
        if (Math.abs(right - left) > 1) {
            return -1;
        }
        return Math.max(right, left) + 1;
    }
}