题解 | #DNA序列#
DNA序列
https://www.nowcoder.com/practice/e8480ed7501640709354db1cc4ffd42a
a = input()
b = int(input())
num_max = sum(1 if i in ['G','C'] else 0 for i in a[0:b])
num = num_max
j = 0
for i in range(0, len(a)-b):
num = num + (1 if a[i+b] in ['G','C'] else 0) - (1 if a[i] in ['G','C'] else 0)
if num > num_max:
num_max = num
j = i + 1
print(a[j:j+b])
