I got a runtime error on the last Atcoder Regular Contest.

The problem is this .

My RE solution here .

for (int k = 1; k < 7; k++) {
	if (!f[k]) {
	    ch[i][j] = (char)(k + '0');
	    break;
	}
}

3 3
...
...
...

I always wrote (char) ('a'+k)to get the kth letter from the alphabet and didn't get Runtime Error on any platform.

Why am I getting RE this time?