两个大数相乘

一、 统一处理进位

-(NSString*)bidnumbstring:(NSString*)muti1 multibyaBignumString:(NSString*)muti2{
    if (muti1.length==0 || muti2.length==0) {
        return @"0";
    }
    
    NSInteger capcity = muti1.length + muti2.length;
    NSMutableArray<NSNumber *> *arr = [NSMutableArray array];
    for (int i = 0; i < capcity; i++) {
        [arr addObject:@0];
    }
    
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wshorten-64-to-32"
    for (int i = muti1.length - 1; i >= 0; i--) {
        for (int j = muti2.length - 1; j >= 0 ; j--) {
            NSInteger singleMuti1 = [[muti1 substringWithRange:NSMakeRange(i, 1)] integerValue];
            NSInteger singleMuti2 = [[muti2 substringWithRange:NSMakeRange(j, 1)] integerValue];

            NSInteger sum = singleMuti1 * singleMuti2;
            NSInteger index = (muti1.length - 1 - i) + (muti2.length - 1 - j);
            arr[index] = @([arr[index] integerValue] + sum);    
    int index = 0;
    while (index < capcity - 1) {
        NSInteger tmp = [arr[index] integerValue];
        arr[index] =  @(tmp % 10);
        arr[index + 1] =  @([arr[index + 1] integerValue] + tmp / 10);
        index++;
    }
    
    NSArray *reverseArr = [[arr reverseObjectEnumerator] allObjects];
    return [reverseArr componentsJoinedByString:@""];
#pragma clang diagnostic pop
}

二、 实时处理进位

-(NSString*)bidnumbstring:(NSString*)muti1 multibyaBignumString:(NSString*)muti2{
    if (muti1.length==0 || muti2.length==0) {
        return @"0";
    }
    
    NSInteger capcity = muti1.length + muti2.length;
    NSMutableArray<NSNumber *> *arr = [NSMutableArray array];
    for (int i = 0; i < capcity; i++) {
        [arr addObject:@0];
    }
    
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wshorten-64-to-32"
    for (int i = muti1.length - 1; i >= 0; i--) {
        for (int j = muti2.length - 1; j >= 0 ; j--) {
            NSInteger singleMuti1 = [[muti1 substringWithRange:NSMakeRange(i, 1)] integerValue];
            NSInteger singleMuti2 = [[muti2 substringWithRange:NSMakeRange(j, 1)] integerValue];

            NSInteger sum = singleMuti1 * singleMuti2;
            NSInteger index = (muti1.length - 1 - i) + (muti2.length - 1 - j);
			do {
                sum += [arr[index] integerValue];
                arr[index] = @(sum % 10);
                if (index + 1 >= capcity) {
                    break;
                }
                arr[index + 1] =  @([arr[index + 1] integerValue] + sum / 10);
                index++;
                sum = 0;
            } while ([arr[index] integerValue] >= 10);
    }
    
    NSArray *reverseArr = [[arr reverseObjectEnumerator] allObjects];
    return [reverseArr componentsJoinedByString:@""];
#pragma clang diagnostic pop
}