题解 | 无限长正整数排列字符串

s = int(input().strip())

def find_pos(s):
    if s <= 9:
        return str(s)
    elif s <= 189:
        s -= 9
        index = (s-1) // 2
        pos = (s-1) % 2
        number = 10 + index
        return str(number)[pos]
    else:
        s -= 189
        index = (s-1) // 3
        pos = (s-1) % 3
        number = 100 + index
        return str(number)[pos]

print(find_pos(s))