View Compilation Error

0_0_20602078_18737.c:3:1: error: expected identifier or '(' before '/' token
 //using namespace std;
 ^
0_0_20602078_18737.c: In function 'dfs':
0_0_20602078_18737.c:9:1: error: expected expression before '/' token
 // printf("%d\n", strlen(use));
 ^
0_0_20602078_18737.c:13:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for (int k = 0; k < i; k++)
  ^
0_0_20602078_18737.c:13:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_20602078_18737.c: In function 'main':
0_0_20602078_18737.c:40:4: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
    for (int k = 0; k < i; k++)
    ^