二分查找

二分查找适用于一个有序的较大数组，核心思想是将查找元素和数组的中间元素作比较，一分为二进行查找，查找速度快
一种易于理解的代码如下：

   function binary_search(arr,key){
        var low=0,high=arr.length-1;//此处为下标11
        while(low<=high){
          var middle=parseInt((low+high)/2);
          if(key===arr[middle]){
            return middle;
          }else if(key>arr[middle]){
            low=middle+1;
          }else if(key<arr[middle]){
            high=middle-1;
          }
        }
        return -1;
      }
      var arr = [1,2,3,4,5,6,7,8,9,10,11,23,44,86];
      var rst1=binary_search(arr,3);
      var rst2=binary_search(arr,100);
      console.log(rst1);//2
      console.log(rst2);//-1