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leetcode旋转数组查找 二分查找的变形

2024-07-11 09:37:17   220人阅读

http://blog.csdn.net/pickless/article/details/9191075

Suppose a sorted array is rotated at some pivot unknown to you beforehand.

(i.e., 0 1 2 4 5 6 7 might become 4 5 6 7 0 1 2).

You are given a target value to search. If found in the array return its index, otherwise return -1.

You may assume no duplicate exists in the array.

设置 low=0  high=len-1

bsearch(A,low,high)=bsearch(A,low,mid-1)||bsearch(A,mid+1,high)   (A[mid]!=target)

                                 mid                 (A[mid]==target)

这是传统的思路：

我们经过观察发现，只要 A[high]>=A[low]  数组一定是递增的，则选用二分查找

  如果不是，则分割 ，使用递归，一定能分割成数组满足第一种状态，

比如，上边的例子我们查找  5，

mid=3,A[mid]=7；分割成（4,5,6）和（0,1,2）连个尾巴都是大于首部，所以直接会被二分查找。

 

 1 public class Solution { 2     public int search(int[] A, int target) { 3         int len=A.length; 4         if(len==0) return -1; 5     return    bsearch(A,target,0,len-1); 6          7          8          9     }10     11     public int bsearch(int A[],int target,int low,int high)12     {13         if(low>high) return -1;   14         int idx=-1;15         if(A[low]<=A[high]) //it must be YouXu,so binary search16         17         {18             while(low<=high)19             {20             int mid=(low+high)>>1;21             if(A[mid]==target) 22             {23                 idx=mid;24                 return idx;25             }26             else if(A[mid]>target)27             {28                 high=mid-1;29             }30             else low=mid+1;31             }32             33         }34         else35         {36             int mid=(low+high)>>1;37             if(A[mid]==target)38             {39              idx=mid;40             return idx;41                 42             }43             else 44             {45                 idx=bsearch(A,target,low,mid-1);46                 if(idx==-1)47                 {48                     idx=bsearch(A,target,mid+1,high);49                     50                 }51                 52                 53                 54                 55             }56             57             58             59             60             61             62         }63         64         return idx;65         66         67     }68     }
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