数组index

　　

1. （Array.length-1）/ 2      and         Array.length/2 取中值 index
   

   Array.length is even
   
   * * *  * * *
       ^
       (Array.length-1)/2
   
   * * *  * * *
          ^
          Array.length/2
   
   当Array.length is even, -1 对中值index有影响
   
   
   Array.length is odd
   
   * *  *  * *
        ^
        (Array.length-1)/2 == Array.length/2
   
   当Array.length is odd, -1 对中值index无影响
   
   example:
   
   1: reverse Array
   for(int i=0;i<array.length/2;++i){
     int tmp=array[array.length-1-i];
     array[array.length-1-i]=array[i];
     array[i]=tmp;
   }