Find Minimum in Rotated Sorted Array

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Last Updated: 15 Jan, 2021

Find Minimum in Rotated Sorted Array

Moderate

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Problem statement

You are given an array 'arr' of size 'n' having unique elements that has been sorted in ascending order and rotated between 1 and 'n' times which is unknown.

The rotation involves shifting every element to the right, with the last element moving to the first position. For example, if 'arr' = [1, 2, 3, 4] was rotated one time, it would become [4, 1, 2, 3].

Your task is to find the minimum number in this array.

Note :

All the elements in the array are distinct.

Example :

Input: arr = [3,4,5,1,2]

Output: 1

Explanation: The original array was [1,2,3,4,5] and it was rotated 3 times.

Input Format :

The first line contains an integer 'n', the size of the array.
The second line contains 'n' space-separated integers.

Output Format :

The only line contains the minimum number in the given array.

Note :

You don’t need to print anything; It has already been taken care of. Just implement the given function.

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Approaches

01Approach

The basic approach to find the minimum element is to traverse the whole array and keep updating the minimum value as we traverse that array.

This is somewhat similar to a linear search. Start by assigning the first element(arr[0]) of the array to a variable named min.
Then iterate the array using a variable i from 1 till n and check if the current element(arr[i]) is less than min. If arr[i] is less than min then assign arr[i] to min.
Now when we come out of this loop, min will give us the minimum element, which is our answer.

Example: 3 4 1 2

Initialize min = 3 and start traversing the array. When we reach 4 we see that 4 > 3 hence we don’t update min. Now we see that 1 < 3, therefore we update min. Now min = 1. Now finally we reach 2 and we see that 2 > 1, hence we don’t update min. Now we have reached the end of the array, hence we print our answer(min), which is equal to 1.

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02Approach

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.

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.
Now we make 4 cases and accordingly solve our problem.
Case 1: arr[low] <= arr[high]
- We return arr[low] since the array is sorted, arr[low] will be the minimum element.
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.
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.
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.
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.

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