answer.code: In function ‘int main()’:
answer.code:10:13: error: ‘strlen’ was not declared in this scope
   10 |         n = strlen(S);
      |             ^~~~~~
answer.code:2:1: note: ‘strlen’ is defined in header ‘<cstring>’; did you forget to ‘#include <cstring>’?
    1 | #include <iostream>
  +++ |+#include <cstring>
    2 | 
answer.code:8:15: warning: ignoring return value of ‘int scanf(const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
    8 |     for (scanf("%d", &_); _ != 0; --_) {
      |          ~~~~~^~~~~~~~~~
answer.code:9:14: warning: ignoring return value of ‘int scanf(const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
    9 |         scanf("%s", S);
      |         ~~~~~^~~~~~~~~