Given a string return all the permutations of that string.

permutations.m

-(NSArray<NSString> *)permutationsOfString:(NSString *)s {
    NSMutableArray<NSString> *res = [NSMutableArray array];
    if (s.length == 1) {
        [res addObject:s];
    } else if (s.length > 1) {
        int lastIndex = s.length - 1;
        String last = s.substring(lastIndex);
        String rest = s.substring(0, lastIndex);
        res = merge(permutation(rest), last);
    }
    return res;
}

-(NSArray<NSString> *)mergeArray:(NSArray<String> *)list withString:(NSString *)c {
    NSMutableArray<NSString> *res = [NSMutableArray array];
    for (NSString *s in list) {
        for (int i = 0; i <= s.length; ++i) {
	    NSMutableString *ps = [NSMutableString stringFromString:s]
            [s insertString:c atIndex:i];
            [res addObject:ps];
        }
    }
    return res;
}