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.

解题思路：

本题求一个树是否是平衡树。可以采用深度优先遍历，计算树的高度，如果左右子树高度差超过1，则返回-1 .

class Solution {
public:
    int isBalancedHelper(TreeNode* root)
    {
        if(root == NULL) return 0;
        int leftHeight = isBalancedHelper(root->left);
        if(leftHeight == -1) return -1;
        
        int rightHeight = isBalancedHelper(root->right);
        if(rightHeight == -1) return -1;
        
        if(abs(leftHeight - rightHeight) > 1) return -1;
        return max(leftHeight,rightHeight) + 1;
    }
    bool isBalanced(TreeNode* root) {
        return isBalancedHelper(root) != -1;
    }
};