110. Balanced Binary Tree

problems:
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.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

      1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.
tips:
判断二叉树是否为高度平衡二叉树，即它的两个子树每个节点的深度之差都不大于1.

solutions:
1.超时，但是逻辑清晰简单，便于理解下一个方法。

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

 class Solution {
    public:
        bool isBalanced(TreeNode* root) {
            return depth(root)!=-1;
        }
        int depth(TreeNode* root)
        {
            if(!root) return 0;
            int left = depth(root->left);
            int right = depth(root->right);
            if(left == -1 || right == -1 || abs(left-right)>1)//不平衡时为-1
                return -1;
            return max(left,right)+1;
        }
    };