View Compilation Error

0_0_31724502_26233.c:4:6: error: variably modified 'arr' at file scope
 char arr[maxn];
      ^
0_0_31724502_26233.c: In function 'main':
0_0_31724502_26233.c:13:9: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
         for(int i=0;i<len-1;i++){
         ^
0_0_31724502_26233.c:13:9: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code