iOS 大数相乘

-(NSString *)bidnumbstring:(NSString*)a multibyaBignumString:(NSString*)b{
    
    if (a.length==0 || b.length==0) {
        return @"0";
    }
     
    
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:a.length+ b.length];
     
    NSMutableArray *n1 = [NSMutableArray arrayWithCapacity:a.length];

    NSMutableArray *n2 = [NSMutableArray arrayWithCapacity:b.length];
    
    //把char转换成int数组。
    
    for (int i =0; i < a.length; i++){
        NSString *abb = [a substringWithRange:NSMakeRange(i,1)];
        n1[i] = @([abb intValue]);
    }
    for (int i =0; i < b.length; i++){
        NSString *bcc = [b substringWithRange:NSMakeRange(i,1)];
        n2[i] = @([bcc intValue]);
    }

    for (int i = 0; i< a.length; i++) {
        for (int j = 0; j< b.length; j++) {
            result[i+j] =@( [n1[i] intValue]  *[ n2[j] intValue]);
        }
    }

    for (int i = result.count- 1; i > 0; i--) {
        result[i-1]  = @( [result[i-1] intValue] + [result[i] intValue] /10);
        result[i] = @( [result[i] intValue] % 10);
    }

    
    NSString* resultStr = @"";

    for (int i = 0; i < result.count - 1; i++) {

         resultStr = [NSString stringWithFormat:@"%@%@",resultStr,result[i]];

    }

    return resultStr;
    
    return  @"";
}