*Search in Rotated Sorted Array II

题目：

 

Follow up for "Search in Rotated Sorted Array":
What if duplicates are allowed?

Would this affect the run-time complexity? How and why?

Write a function to determine if a given target is in the array.

 

解题思路：

This problem is similar to “Search in Rotated Sorted Array”. But there could be duplicates.

In problem “Search in Rotated Sorted Array”, we compare A[left] with A[mid] to determine which part does not contains the rotate pivot. But in this problem, A[left] could be equal to A[mid]. To solve this problem, we can let just add 1 to left when we face A[left] == A[mid] and continue the algorithm.

 1 public class Solution {

 2     public boolean search(int[] A, int target) {

 3         int start = 0;

 4         int end = A.length - 1;

 5         while (start <= end) {

 6             int mid = (start + end) / 2;

 7             if (A[mid] == target)

 8                 return true;

 9             if (A[start] < A[mid]) {

10                 if (target >= A[start] && target < A[mid])

11                     end = mid - 1;

12                 else

13                     start = mid + 1;

14             } else if (A[start] > A[mid]) {

15                 if (target > A[mid] && target <= A[end])  //注意边界条件

16                     start = mid + 1;

17                 else

18                     end = mid - 1;

19             } else 20 start++; 21         }

22         return false;

23     }

24 }

Complexity

The worst case complexity will become O(n). 

和Search in Rotated Sorted Array唯一的区别是这道题目中元素会有重复的情况出现。不过正是因为这个条件的出现，出现了比较复杂的case，甚至影响到了算法的时间复杂度。原来我们是依靠中间和边缘元素的大小关系，来判断哪一半是不受rotate影响，仍然有序的。而现在因为重复的出现，如果我们遇到中间和边缘相等的情况，我们就丢失了哪边有序的信息，因为哪边都有可能是有序的结果。假设原数组是{1,2,3,3,3,3,3}，那么旋转之后有可能是{3,3,3,3,3,1,2}，或者{3,1,2,3,3,3,3}，这样的我们判断左边缘和中心的时候都是3，如果我们要寻找1或者2，我们并不知道应该跳向哪一半。解决的办法只能是对边缘移动一步，直到边缘和中间不在相等或者相遇，这就导致了会有不能切去一半的可能。所以最坏情况（比如全部都是一个元素，或者只有一个元素不同于其他元素，而他就在最后一个）就会出现每次移动一步，总共是n步，算法的时间复杂度变成O(n)。

 

reference： 

http://www.lifeincode.net/programming/leetcode-search-in-rotated-sorted-array-ii-java/

http://www.cnblogs.com/springfor/p/3859525.html

http://blog.csdn.net/linhuanmars/article/details/20588511