View Compilation Error

0_0_13843856_10014.c: In function 'main':
0_0_13843856_10014.c:28:2: error: expected expression before '/' token
  //freopen("in.txt", "r", stdin);
  ^
0_0_13843856_10014.c:34:10: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
          for(int j = 2 * n; j >= 0; --j)
          ^
0_0_13843856_10014.c:34:10: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code