** Leetcode 687. Longest Univalue Path | 一次dfs做两个事情 path sum

https://leetcode.com/problems/longest-univalue-path/description/

哟意思和有点难度的地方 在一次dfs做两个事情

/**
 * 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:
    //返回值 和当前结点一样的左子树
    int dfs( TreeNode* root, int &cur_ans) {
        if (!root) return 0;
        
        int lcnt = 0, rcnt = 0;
        if (root->left) {
            if (root->left->val == root->val) {
                lcnt = dfs(root->left, cur_ans);
            } else {
                dfs(root->left, cur_ans);
            }
        }
        if (root->right) {
            if (root->right->val == root->val) {
                rcnt = dfs(root->right, cur_ans);
            } else {
                dfs(root->right, cur_ans);
            }
        }
            
        cur_ans = max(cur_ans, lcnt + rcnt + 1);
        int ret = max(lcnt, rcnt) + 1;
        cur_ans = max(cur_ans, ret);
        return ret;
    }
    
    int longestUnivaluePath(TreeNode* root) {
        if (!root) return 0;
        int child_ans = 0;
        int ret = dfs(root, child_ans);
        return max(ret, child_ans) - 1;
    }
};