A string problem for collating 2 strings together. Strings "abcdef" and "123456" become "a1b2c3d4e5f6."

main.m

#import "NSString+Collate.h"

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSArray *collection = @[
                                @[@"a", @"1"],
                                @[@"ab", @"12"],
                                @[@"abcdef", @"123456"],
                                @[@"ab", @"1234"],
                                @[@"abcd", @"12"],
                                @[@"abc", @"abc"],
                                @[@"123", @"123"]
                                ];

        NSUInteger i = 1;
        for (NSArray *strings in collection) {
            NSString *str1 = strings[0];
            NSString *str2 = strings[1];
            NSLog(@"%lu: %@", i, [str1 collateWithOtherString:str2]);
            i++;
        }
    }
    return 0;
}

NSString+Collate.h

@interface NSString (Collate)

- (NSString *)collateWithOtherString:(NSString *)otherString;

@end

NSString+Collate.m

@implementation NSString (Collate)

- (NSString *)collateWithOtherString:(NSString *)otherString {
    return [self collateString1:self string2:otherString];
}

- (NSString *)collateString1:(NSString *)str1 string2:(NSString *)str2 {
    NSMutableString *result = [@"" mutableCopy];
    for (NSUInteger index=0; (index<str1.length || index<str2.length); index++) {
        if (index < str1.length) {
            [result appendString:[str1 substringWithRange:NSMakeRange(index, 1)]];
        }
        if (index < str2.length) {
            [result appendString:[str2 substringWithRange:NSMakeRange(index, 1)]];
        }
    }
    
    return result;
}

@end