Jun 17, 2013

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Find minimum element in rotated and sorted array.


Problem : We have to find an element which is having lowest value in the given array which is sorted and rotated. FYI: This question was asked me in oracle interview.(All elements of the array are unique)
what is sorted and rotated array ?
Sorted array (ONLY) :

Sorted and rotated array (BOTH): 
                                   
By brute force approach we can find that element in O(n) , off course this is not expected answer
any interviewer would like to hear at least not in F2F interview  :P
So what next ?  Since we have a sorted array  and we have to find a solution better than O(n) so Binary search may be handy to solve this problem. If we see diagram closely we can find that  main rationale behind this problem is :
 If (array is rotated )   
    lowest element is that element whose previous element is always higher   
 else  
    first element is lowest element.  
Algorithm : 
  Do simple binary search  
       find middleIndex from startIndex and endIndex of array  
        if (element@middleIndex > element@(middleIndex+1))   
           lowest element is element@(middle+1)  
           return "we are done!!"  
        else if (element@middleIndex < element@(middleIndex-1))  
            lowest element is element@(middle)  
             return "we are done!!"  
        else   
           if(element@middleIndex < element@endIndex)  
             Do not move in right direction , since elements are in increasing order.  
             go to left direction and  repeat binary search methd   
           else   
             Do not move in left direction , since elements are in increasing order.  
             go to right direction and  repeat binary search methd 
       
Now we will write the sample java code for this .
 public class FindMinimumInRotatedArray {  
   public FindMinimumInRotatedArray() {  
     super();  
   }  
 private int findMinInSortedAndRotated(int arr[], int low, int high)  
 {  
   // Added to handle the case when array is not  
   // rotated at all  
   if (high < low) return arr[0];  
   // If there is only one element left  
   if (high == low) return arr[low];  
   // Find mid  
   int mid = low + (high - low)/2; /*To avoid overflow do not use (low + high)/2;*/  
   if (mid < high && arr[mid+1] < arr[mid])  
     return arr[mid+1];  
   // Check if mid itself is minimum element  
   if (mid > low && arr[mid] < arr[mid - 1])  
     return arr[mid];  
   // Decide whether we need to go to left half or right half  
   if (arr[high] > arr[mid])  
     return findMin(arr, low, mid-1);  
   return findMin(arr, mid+1, high);  
 }  
   public static void main(String[] args) {  
     FindMinimumInRotatedArray findMinimumInRotatedArray = new FindMinimumInRotatedArray();  
     int[] arr = { 6, 7, 8, 9, 1, 2, 3, 4, 5 };  
     System.out.println(findMinimumInRotatedArray.findMinInSortedAndRotated(arr, 0, 9));  
     }  
 }  
Sample output : 1

when we execute program we will get 1 output , since it is minimum element.

Now if we twist the question little bit what will happen if all elements of the array are not unique.
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Happy learning!!
Nikhil