/in/foo.cc: In function 'int main()':
/in/foo.cc:12:16: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'char (*)[5001]' [-Wformat=]
   scanf("%s",&a); // 读入序列，存入数组a
                ^
/in/foo.cc:14:31: error: 'i' was not declared in this scope
   for (int j = 0; j <= n - 1; i++){
                               ^