[leetcode] 250. Count Univalue Subtrees 解题报告

题目链接: https://leetcode.com/problems/count-univalue-subtrees/

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,

              5
             / \
            1   5
           / \   \
          5   5   5

return 4.

思路: 题意是说如果一个结点的左子树和右子树的值都相等, 那么就计数+1. 因此我们只需要一个递归的判定一个结点出发,是否其所有结点都相等即可.叶子结点也算是一个结果.

代码如下:

/**
 * 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 DFS(TreeNode* root, int pre, int& ans)
    {
        if(!root) return true;
        bool flag1 = DFS(root->left, root->val, ans);
        bool flag2 = DFS(root->right, root->val, ans);
        if(flag1 && flag2) ans++;
        return (root->val == pre) && flag1 && flag2;
    }
    
    int countUnivalSubtrees(TreeNode* root) {
        if(!root) return 0;
        int ans = 0;
        DFS(root, root->val, ans);
        return ans;
    }
};