977A - Wrong Subtraction http://codeforces.com/contest/977/problem/A

簡單題 題意如果尾數為0 除10 否則-1

for (int i = 0; i < k; ++i) 

{
		if (n % 10 == 0) n /= 10;
		else n--;
}

977B - Two-gram http://codeforces.com/contest/977/problem/B

遍歷所有子串並計算每個的次數 或 所有二元數字的次數

for (int i = 0; i < n - 1; ++i) {
		int cur = 0;
		for (int j = 0; j < n - 1; ++j)
			if (s[j] == s[i] && s[j + 1] == s[i + 1])
				++cur;
		if (res < cur) {
			res = cur;
			ans = string(1, s[i]) + string(1, s[i + 1]);
		}
	}

977C - Less or Equal  http://codeforces.com/contest/977/problem/C
排個序 取出那個第k個數ans 比較一下 如果小於等於ans的數不等於k輸出 -1
其實就是第k個數是否只在k位置 不是就輸出-1
 

D. Divide by three, multiply by two http://codeforces.com/contest/977/problem/D

將數分成3^a+b a不相同時 兩個數為二倍關系

按a大小排序 同大小a按b大到小輸出 同理輸出a-1的

 for(int i=0; i<n; i++)
    printf("%lld%c", v[i].second, " \n"[i + 1 == n]);
}

Codeforces Round #479 (Div. 3)