字符串从小到大排列

-(NSString *)sortString:(NSString *)str{

          NSMutableString *muString = [NSMutableString stringWithString:str];

         //冒泡排序

         for (int i = 0; i < muString.length - 1; i++) {

         for (int j = 0; j < muString.length - i - 1; j++) {

         if ([muString characterAtIndex:j] > [muString characterAtIndex:j+1]) {

         //把第j+1的值取出来赋值给p

         unichar p = [muString characterAtIndex: j+1];

        //用第j为替换第j+1位

       [muString replaceCharactersInRange:NSMakeRange(j+1, 1) withString:[NSString       stringWithFormat:@"%c",[muString characterAtIndex:j]] ];

        //用第j+1位的值替换给第j位，完成交换

        [muString replaceCharactersInRange:NSMakeRange(j, 1) withString:[NSString  stringWithFormat:@"%c",p]];

                    }

          }

    }

return muString;

}