LeetCode_687

class Solution {
public:
    int longestUnivaluePath(TreeNode* root) {
        if(root == nullptr) return 0;
        int ans = 0;
        univalue(root, &ans);
        return ans;
    }
private:
    int univalue(TreeNode& root, int* ans) {
        if(root == nullptr) return 0;
        int l = univalue(root->left, ans);
        int r = univalue(root->right, ans);

        int pl = 0;
        int pr = 0;

        if(root->left && root.val == root->left.val) pl = l + 1;
        if(root->left && root.val == root->right.val) pr = r + 1;

        *ans = max(*ans, pr + pl);
        return max(pl,pr);
    }
};