Rotated Array — Asked in Ola and Facebook Interview

Placewit
3 min readAug 6, 2021

Problem Statement :

You’re given a sorted array that has now been rotated ‘K’ times, which is unknown to you. Rotation here means that every element is shifted from its position to right in each rotation and the last element simply shifts to the first position. For example: 1 2 3 4, after one rotation becomes 4 1 2 3. Your task is to find the minimum number in this array.

Sample Input:

N = 4
Arr = {3 4 1 2}

Output:

1

Explanation of given Test Cases :

As it can be clearly seen that our initial sorted array was 1 2 3 4 and after one rotation it became 4 1 2 3 and after the second rotation it became 3 4 1 2, which is given in the input, and here 1 is the minimum element.

Approach :

Optimized Approach

As it is given in the hint itself, we want to decrease the search space in order to optimize our solution. Hence we want to apply the technique of divide and conquer for this. Here use a binary search algorithm to do so.

  1. We start by making two variables named low and high, where low = 0 and high = n-1. Now we run a loop that will run till low <= high. We also make another variable mid, which is equal to (low+high)/2.
  2. Now we make 4 cases and accordingly solve our problem.
  3. Case 1: arr[low] <= arr[high]
  • We return arr[low] since the array is sorted, arr[low] will be the minimum element.

4. Now we make a variable named next which will be equal to (mid+1)%n and previous which will be equal to (mid+n-1)%n.

5. Case 2: (arr[mid] <= arr[next]) and (arr[mid] <= arr[previous])

  • We return arr[mid]. We do this because on observing we can see that only the minimum element will have this special property where it is smaller than both of its neighbours. Example 4 1 2 3, here you can see that only 1, which is the minimum element, in this case, is the element that is smaller than both of its neighbours 4 and 2.

6. Case 3: arr[mid] <= arr[high]

  • Here we see that if the current middle element is less than the current high element, that means the minimum element won’t be present in the right half of the array(with reference to mid). Hence we discard this search space and make high = mid-1.

7. Case 4: arr[mid] >= arr[low]

  • This case is similar to Case 3, the only difference is that here we can observe that if our current middle element is greater than the current lower element, the minimum element won’t be present in the left half of the array(with reference to mid). Hence we discard this search space and make low = mid+1.

Time Complexity: O(log(N))
Space Complexity: O(1)

Code :

Thanks for Reading

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