http://www.lintcode.com/en/problem/balanced-binary-tree/ https://leetcode.com/problems/balanced-binary-tree/description/

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

The binary tree A is a height-balanced binary tree, but B is not.

解题 ：要求平衡二叉树，所谓平衡二叉树就是叶子节点高度差不大于1。我们用分治法，遇到null时候返回0，然后比较左右子树高度差是不是大于1，当然也要看左右子树是不是－1，需要把－1冒泡到顶层去。

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: True if this Binary tree is Balanced, or false.
     */
    public boolean isBalanced(TreeNode root) {
        // write your code here
        int r = 0;
        if(root != null) {
            r = helper(root);
        }
        if(r == -1) {
            return false;
        }
        return true;
    }

    public int helper(TreeNode node) {
        if(node == null) {
            return 0;
        }

        int leftDepth = helper(node.left);
        int rightDepth = helper(node.right);

        if(leftDepth == -1 || rightDepth == -1 || Math.abs(leftDepth - rightDepth) > 1) {
            return -1;
        }
        return Math.max(leftDepth, rightDepth) + 1;
    }
}