[leetcode]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.

思路很简单，对于每个节点，递归求得其左右子节点的高度。由此可以得到它是否是平衡二叉树。对于其左右子节点用同样的方法递归。当所有的节点都是平衡二叉树的时候，那么整棵树才是平衡二叉树。

 

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool key;
    bool isBalanced(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        key = true;
        DepthFirst(root);
        return key;
    }
    int DepthFirst(TreeNode *root)
    {
        if(root == NULL)
            return 0;
        int lDepth = DepthFirst(root->left);
        if(key == false)
            return -1;
        int rDepth = DepthFirst(root->right);
        if(key == false)
            return -1;
        if(lDepth > rDepth + 1 ||rDepth  > lDepth + 1)
        {
            key = false;
            return -1;
        }
        int max = (lDepth >= rDepth)?lDepth:rDepth;
        return 1 + max;
    }
};