LeetCode: 110. 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.

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.

题目大意： 判断给定二叉树是否平衡。

解题思路 —— 递归求解

递归判断左子树和右子树的高度差是否超过 1.

AC 代码

/** * 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 {
private:
    // 如果返回值为非负，表示该二叉树是平衡的；否则表示其不平衡
    int CalHeight(TreeNode* root)
    {
        if(root == nullptr) return 0;

        int leftChildHeight = CalHeight(root->left);
        // 左子树不平衡
        if(leftChildHeight == -1) return -1;

        int rightChildHeight = CalHeight(root->right);
        // 右子树不平衡
        if(rightChildHeight == -1) return -1;

        if(abs(leftChildHeight - rightChildHeight) > 1)
        {
            return -1; // 不平衡
        }

        return max(leftChildHeight, rightChildHeight) + 1;
    }
public:
    bool isBalanced(TreeNode* root) {
        return (CalHeight(root) >= 0);
    }
};