（树）687. Longest Univalue Path

最近在做树结构的相关习题，因为之前随机抽题的时候发现这个模块总是出错，考虑的不周全，所以想集中做一下。

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

题目要求为返回二叉树中路径节点值相等的最大路径长度。我在第一遍做的时候，仅想通过递归返回最大路径，结果又与子递归中应返回的结果相矛盾，思路很乱。参考他人代码时才发现只有设置一个全局变量，将最大路径与递归返回结果分开就可以解决这个问题。目前做的题还是太少，没有形成一个固定的思维模式。

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:

int longestpath;

    int longestUnivaluePath(TreeNode* root) {

        

        longestpath= 0;

if(root) dfs(root);

return longestpath;

    

    }

    

  int dfs(TreeNode* root)

{

int left_branch = root->left ? dfs(root->left) : 0;

int right_branch = root->right ? dfs(root->right) : 0;

int sum_left_branch = root->left && root->left->val == root->val ? (left_branch+1) : 0;

int sum_right_branch = root->right && root->right->val == root->val ? (right_branch+1) : 0;

longestpath = max(longestpath, sum_left_branch + sum_right_branch);

return max(sum_left_branch, sum_right_branch);

}

    

    

};