LeetCode687. Longest Univalue Path

LeetCode687. Longest Univalue Path

题目：

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.

Example 1:

Input:

5
/ \
4   5
/ \   \
1   1   5

Output:

2

Example 2:

Input:

1
/ \
4   5
/ \   \
4   4   5

Output:

2

题目分析：
大部分的树第一步的想法就是递归，或者用BFS的方法实现。所以这道题首先想的也是往递归的方向去思考。
如何递归，首先得找到终止条件，找到子问题，有点类似动态规划问题。

以下是我的代码：

class Solution {
public:
int helper(TreeNode* root, int val)
{
if(!root || root->val != val) return 0;
return 1 + max(helper(root->left,val),helper(root->right,val));
}
int longestUnivaluePath(TreeNode* root) {
if(!root) return 0;
int sub = max(longestUnivaluePath(root->left),longestUnivaluePath(root->right));
return max(sub,helper(root->left,root->val) + helper(root->right,root->val));
}
};