LeetCode 687. Longest Univalue Path(相同节点值的最大路径长度)

题意：统计相同节点值的最大路径长度，两点间的最大路径代表两点间边的个数。

/**
 * 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 ans;
    int dfs(TreeNode* root){
        if(root == NULL) return 0;
        int left = dfs(root -> left);
        int right = dfs(root -> right);
        if(root -> left && root -> val == root -> left -> val){
            ++left;
        }
        else{
           left = 0;
        }
        if(root -> right && root -> val == root -> right -> val){
            ++right;
        }
        else{
            right = 0;
        }
        ans = max(ans, left + right);
        return max(left, right);
    }
    int longestUnivaluePath(TreeNode* root) {
        ans = 0;
        dfs(root);
        return ans;
    }
};