Check out this video editorials for codeforces [Codeforces Round #646 (Div. 2)]

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problem B : Problem - B - Codeforces
video Link :B. Subsequence Hate | Codeforces Round #646 (Div. 2) - YouTube

problem C : Problem - C - Codeforces
video Link :C. Game On Leaves | Codeforces Round #646 (Div. 2) - YouTube