iOS 随机字符串方法

//产生length个长度随机字符串
- (NSString *)getRandStringWithLength:(int)length {
    NSString *sourceStr = @"0123456789abcdefghijklmnopqrstuvwxyz";
    NSMutableString *resultStr = [[NSMutableString alloc] init];   
    for (int i = 0; i < length; i++) {
        unsigned index = arc4random() % [sourceStr length];
        NSString *oneStr = [sourceStr substringWithRange:NSMakeRange(index, 1)];
        [resultStr appendString:oneStr];
    }
    return resultStr;
}