題目描述(Easy)

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.

題目連結

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.

演算法分析

遞迴計算左右子樹的深度，-1代表非平衡樹。

提交程式碼：

/**
 * 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) {
        return calTreeHeight(root) >= 0;
    }
    
    int calTreeHeight(TreeNode* root)
    {
        if (root == NULL) return 0;
        
        int left = calTreeHeight(root->left);
        int right = calTreeHeight(root->right);
        
        if (left < 0 || right < 0 || abs(left - right) > 1) return -1;
        
        return max(left, right) + 1;
    }
};