Checking if two strings are permutations of each other

Question

How to determine if two strings are permutations of each other

Answer 1

This is an O(N) solution, in which N is the length of the shorter string. It has a disadvantage, which is that only ASCII characters are acceptable. If we want better applicability, we may substitute a hash table for the int charNums[]. But it also means C wouldn't be a good choice, coz' there is no standard hash table implementation for C.

int is_permutation(char *s1, char *s2)
{
    if ((NULL == s1) ||
        (NULL == s2)) {
        return false;
    }

    int static const
    _capacity = 256;    // Assumption
    int
    charNums[_capacity] = {0};
    char
    *cRef1 = s1,
    *cRef2 = s2;
    while ('\0' != *cRef1 && '\0' != *cRef2) {
        charNums[*cRef1] += 1;
        charNums[*cRef2] -= 1;
        cRef1++;
        cRef2++;
    }

    if ('\0' != *cRef1 || '\0'  != *cRef2) {
        return false;
    }

    for (int i = 0; i < _capacity; i++) {
        if (0 != charNums[i]) {
            return false;
        }
    }

    return true;
}