Easy
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.

submit 1:

Screen Shot 2017-09-02 at 9.23.01 AM.png

AC:

Screen Shot 2017-09-02 at 9.21.14 AM.png

最后的判断条件是该节点的左子树右子树高度差不超过1， 并且左子树和右子树也都是 Balanced Binary Tree.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null){
            return true;
        }
        int diff =  Math.abs(helper(root.left) - helper(root.right));
        if (diff > 1){
            return false;
        }
        return isBalanced(root.left) && isBalanced(root.right);
    }
    
    private int helper(TreeNode head){
        if (head == null){
            return 0;
        }
        if (head.left == null && head.right == null){
            return 1;
        }
        return Math.max(helper(head.left), helper(head.right)) + 1;
    }
}