九.查找算法

一.二分查找

1.非递归实现

索引 while

left<=right

 1     public static int binarySearch(int[] arr,int key) {
 2         int left=0;
 3         int right=arr.length-1;
 4         while(left<=right) {        // 小于等于<=
 5             int mid=(left+right)/2;
 6             if(key==arr[mid])
 7                 return mid;
 8             else if(key<arr[mid]) 
 9                 right=mid-1;
10             else 
11                 left=mid+1;
12         }
13         return -1;                    //没找到返回-1
14     }

2.递归实现（很有参考价值）

 1     public static int binarySearch2(int[] arr,int key,int left,int right) {
 2         int mid=(left+right)/2;
 3         
 4         if(left>right)    //递归结束条件
 5             return -1;
 6         if(key==arr[mid])
 7             return mid;
 8         else if(key<arr[mid]) 
 9             return binarySearch2(arr, key,left,mid-1);
10         else
11             return binarySearch2(arr, key, mid+1, right);
12     }