题解 | #DNA序列#

滑动数组的方法，从0位开始遍历，子串末尾是"G"or"C",则计数+1；
子串数组滑动到下一位之前，如果子串首位是"G"or"C",则计数-1

while True:
    try:
        s = input()
        L = int(input())
        
        # sub = deque([], L)
        sub = []  # len = L
        GC_i, tmp, res = 0, 0, -1
        for i in range(len(s)):
            sub.append(s[i])
            if sub[-1] == 'G' or sub[-1] == 'C':
                tmp += 1
                
            if len(sub) == L:
                if tmp > res:
                    res = tmp
                    GC_i = i - L + 1
                if sub[0] == 'G' or sub[0] == 'C':
                    tmp -= 1
                sub.pop(0)
                
        print(s[GC_i: GC_i+L])
    except:
        break