05-19 Codeforces Round #639 (Div. 2) 题解 05-19 Codeforces Round #640 (Div. 4) 题解 05-19 AtCoder Beginner Contest 167 题解 05-19 Codeforces Round #642 (Div. 3) 题解 05-19 Codeforces Round #643 (Div. 2) 题解