NSString 字符串的合并
NSString* string; // 结果字符串
NSString* string1, string2; //已存在的字符串
下面这三种方法可以让字符串合并：
string = [NSString initWithFormat:@"%@,%@", string1, string2 ];
string = [string1 stringByAppendingString:string2];
string = [string stringByAppendingFormat:@"%@,%@",string1, string2];
第二个方法更有效率。
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