#93 Balanced Binary Tree

题目描述：

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.

Have you met this question in a real interview? 

Yes

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.

题目思路：

这题就用recursion做，然后我还算了每个node的height，返回true的条件是height之差小于等于1并且左子树和右子树都balance。

Mycode（AC = 13ms）：

/**
* Definition of TreeNode:
* class TreeNode {
* public:
* int val;
* TreeNode *left, *right;
* TreeNode(int val) {
* this->val = val;
* this->left = this->right = NULL;
* }
* }
*/
class Solution {
public:
/**
* @param root: The root of binary tree.
* @return: True if this Binary tree is Balanced, or false.
*/
bool isBalanced(TreeNode *root) {
// write your code here
if (root == NULL) {
return true;
}

return abs(height(root->left) - height(root->right)) <= 1 &&
isBalanced(root->left) && isBalanced(root->right);
}

int height(TreeNode *root) {
if (root == NULL) {
return 0;
}

return max(height(root->left), height(root->right)) + 1;
}
};