[leetcode] 250. Count Univalue Subtrees 解题报告

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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;    }};

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