Balanced Binary Tree

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.

判断一颗树是否是平衡树。我们先判断根节点左右子树的深度，比较它们的高度差，如果大于1就返回false，如果小于1，递归判断左右子树是否为平衡树。代码如下：

/**
 * Definition for a binary tree node.
 * 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;
        int left = getDepth(root.left, 1);
        int right = getDepth(root.right, 1);
        if(left - right > 1 || left - right < -1) {
            return false;
        } else {
            return isBalanced(root.left) && isBalanced(root.right);
        }
    }
    public int getDepth(TreeNode root, int depth) {
        if(root == null) return depth;
        return Math.max(getDepth(root.left, depth + 1), getDepth(root.right, depth + 1));
    }
}