leetcode 110

110. 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.

判断二叉树是否为平衡二叉树。

递归解法：
（1）如果二叉树为空，返回真
（2）如果二叉树不为空，如果左子树和右子树都是AVL树并且左子树和右子树高度相差不大于1，返回真，其他返回假

代码如下：

/**
 * 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) {
        int height;
        return isAvl(root, height);
    }
    bool isAvl(TreeNode* pRoot, int & height)
    {
        if(pRoot == NULL)
        {
            height = 0;
            return true;
        }
        int heightLeft;
        bool resultLeft = isAvl(pRoot->left, heightLeft);
        int heightRight;
        bool resultRight = isAvl(pRoot->right, heightRight);
        if(resultLeft && resultRight && abs(heightLeft - heightRight) <= 1)
        {
            height = max(heightLeft, heightRight) + 1;
            return true;
        }
        else
        {
            height = max(heightLeft, heightRight) + 1;
            return false;
        }
    }
};