题目:

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.

题目解答：判断一棵给定的二叉树是不是平衡二叉树。平衡二叉树的条件有：

　　　　它是一棵空树或它的左右两个子树的高度差的绝对值不超过1，并且左右两个子树都是一棵平衡二叉树。

对于树的处理，一般都使用递归的方式。

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        if(root == NULL)
            return true;
        if((root -> left == NULL ) && (root -> right == NULL))
            return true;
        int leftDepth = 0;
        int rightDepth = 0;
        return isSubBalance(root -> left,leftDepth) && isSubBalance(root -> right , rightDepth) && (abs(leftDepth - rightDepth) <= 1) ;
    }
   
    bool isSubBalance(TreeNode *root,int &depth)
    {
        if(root == NULL)
        {
            depth = 0;
            return true;
        }
        if((root -> left == NULL ) && (root -> right == NULL))
        {
            depth = 1;
            return true;
        }
        else
        {
            depth = 1;
            int leftDepth = 0;
            int rightDepth = 0;
            return isSubBalance(root -> left,leftDepth) && isSubBalance(root -> right , rightDepth) && (abs(leftDepth - rightDepth) <= 1) && (depth += max(leftDepth,rightDepth) ) ;
        }
    }
   
};