Find Maximum and Minimum Element in an Array

Difficulty: Medium, Asked-In: Facebook, Microsoft.

Key Takeaways

  • An excellent problem to learn problem-solving using a single loop and divide-and-conquer approach.
  • We are incrementing the loop by two (a constant) to optimize the code further. We can use similar ideas to optimize the code for other coding problems.
  • The base conditions, initializations, and conditional statements are intuitive. We can find similar patterns in other coding problems.
  • The time complexities of all three approaches are O(n), but the total count of comparison operations is different. Such a scenario may arise in solving other coding problems where one can reduce the count of critical operations to optimize the solution further.

Let’s understand the problem

Given an array X[] of size n, we need to find the maximum and minimum elements present in the array. Our algorithm should make the minimum number of comparisons.

Examples

Input: X[] = [4, 2, 0, 8, 20, 9, 2], Output: max = 20, min = 0

Input: X[] = [-8, -3, -10, -32, -1], Output: max = -1, min = -32

Important Note: Before moving on to the solutions, we recommend learners solve this problem on paper. If solved, well done! We would love to hear your ideas in the message below. Otherwise, no problem! Consider this an excellent opportunity to learn a new pattern in problem-solving.

Follow-up questions

Candidate: Are the input values unique?

Interviewer: For the convenience of the solution, we can assume that they are.

Candidate: Do we need to solve this problem in place?

Interviewer: Yes, we are looking for an O(n) in-place solution with the minimum number of comparison counts.

Now, we are moving forward and discussing the solution ideas step-by-step. Practicing these steps could help us arrive at an efficient solution during a coding interview.

Discussed solution approaches

  • Brute force approach using single loop: Increment by 1
  • Using divide and conquer approach similar to merge sort
  • Efficient approach using single loop: Increment by 2

Brute force solution using single loop: Increment by 1

Solution idea and steps

Step 1: We initialize two variables, max and min, with X[0] to store the maximum and minimum.

Step 2: Now we traverse the array from i = 1 to n - 1 and compare each element with min and max.

  • If (X[i] < min): We have found a value X[i] smaller than the minimum so far. So, we update min with X[i], i.e., min = X[i].
  • If (X[i] > max): We have found a value X[i] greater than the maximum so far. So, we update max with X[i], i.e., max = X[i].
  • By the end of the loop, the minimum and maximum values of the array will be stored in the variables min and max.

Step 3: To return these values, we create an extra array maxMin[] of size two, where we store the maximum at the first index and the minimum at the second index. We return the maxMin array as output.

Solution pseudocode

int[] findMinMax(int X[], int n)
{
    int max = X[0]
    int min = X[0]
    for (int i = 1; i < n; i = i + 1)
    {
        if (X[i] > max)
            max = X[i]
        else if (X[i] < min)
            min = X[i]
    }
    int maxMin[2] = {max, min}
    return maxMin
}

Python implementation

def findMinMax(X, n):
    max = X[0]
    min = X[0]
    for i in range(1, n):
        if X[i] > max:
            max = X[i]
        elif X[i] < min:
            min = X[i]
    maxMin = [max, min]
    return maxMin

Solution analysis

We are running a single loop n - 1 time and doing O(1) operations at each iteration. So time complexity = (n - 1)*O(1) = O(n). We are using constant extra space, so space complexity = O(1).

The critical questions are: What would be the worst and best-case scenario? Comparison is a critical operation in the code. So what would be the comparison count in the worst and best case? Let's think!

In the worst case, we make two comparisons at each step of the iteration. This case will arise if the array is sorted in descending order. In this situation, the first if statement will be false every time, and the second if statement will be true every time. So the total number of comparisons in the worst case = 2*(n - 1) = 2n - 2.

The best case occurs when elements are sorted in ascending order. In this situation, a total of n - 1 comparisons will be made.

Using divide and conquer approach similar to merge sort

Solution idea

Now the critical questions are: Can we solve this problem using another approach? Can we think recursively to find maximum and minimum values efficiently? Let's think!

If we divide the array into two equal parts and find the minimum and maximum of both halves recursively, we can easily find the maximum and minimum of the overall array. For this, we compare the minimum of both halves to get the overall minimum and the maximum of both halves to get the overall maximum. This looks like a divide and conquer idea, similar to the merge sort algorithm!

Divide step: We divide the array into two equal parts around the mid index, i.e., we divide the problem into two equal-sized subproblems.

Conquer step: We recursively find the minimum and maximum of the left and right parts.

Combine step: We compare the maximum of both halves to get the overall maximum and the minimum of both halves to get the overall minimum.

Solution steps

We write a recursive function that accepts the array and its start and end indices as input parameters, i.e., findMinMax(int X[], int l, int r).

Base case 1: If the array size is 1, we return that single element as both the maximum and minimum.

Base case 2: If the array size is 2, we compare both elements and return the maximum and minimum.

if (l == r)
{
        max = X[l]
        min = X[l]
}

else if (l + 1 == r)
{
    if (X[l] < X[r])
    {
        max = X[r]
        min = X[l]
    }
    else
    {
        max = X[l]
        min = X[r]
    }
}

Divide part: We calculate the mid index i.e. mid = l + (r - l)/2.

Conquer part

  1. We recursively calculate and store the maximum and minimum for the left part, i.e., leftMinMax[2] = findMinMax(X, l, mid).
  2. We recursively calculate and store the maximum and minimum for the right part, i.e., rightMinMax[2] = findMinMax(X, mid + 1, r).

Combine part: Now we find the overall maximum and minimum by comparing the min and max of both halves. For this, we need to perform two comparisons only.

if (leftMinMax[0] > rightMinMax[0])
    max = leftMinMax[0]
else
    max = rightMinMax[0]
    
if (leftMinMax[1] < rightMinMax[1])
    min = leftMinMax[1]
else
    min = rightMinMax[1]

Finally, we store max and min in extra memory maxMin[2] and return it.

Solution pseudocode

int[] findMinMax(int X[], int l, int r)
{
    int max, min
    if (l == r)
    {
        max = X[l]
        min = X[l]
    }
    else if (l + 1 == r)
    {
        if (X[l] < X[r])
        {
            max = X[r]
            min = X[l]
        }
        else
        {
            max = X[l]
            min = X[r]
        }
    }
    else
    {
        int mid = l + (r - l)/2
        int leftMinMax[2] = findMinMax(X, l, mid)
        int rightMinMax[2] = findMinMax(X, mid + 1, r)
        if (leftMinMax[0] > rightMinMax[0])
            max = leftMinMax[0]
        else
            max = rightMinMax[0]
        if (leftMinMax[1] < rightMinMax[1])
            min = leftMinMax[1]
        else
            min = rightMinMax[1]
    }
    int maxMin[2] = {max, min}
    return maxMin
}

Python implementation

def findMinMax(X, l, r):
    max, min = 0, 0
    if l == r:
        max = X[l]
        min = X[l]
    elif l + 1 == r:
        if X[l] < X[r]:
            max = X[r]
            min = X[l]
        else:
            max = X[l]
            min = X[r]
    else:
        mid = l + (r - l) // 2
        leftMinMax = findMinMax(X, l, mid)
        rightMinMax = findMinMax(X, mid + 1, r)
        if leftMinMax[0] > rightMinMax[0]:
            max = leftMinMax[0]
        else:
            max = rightMinMax[0]
        if leftMinMax[1] < rightMinMax[1]:
            min = leftMinMax[1]
        else:
            min = rightMinMax[1]
    maxMin = [max, min]
    return maxMin

Solution analysis

This is a recursive solution. So we need to define the recurrence relation to analyze time complexity. Suppose T(n) is the time complexity of problem size n.

  • We are dividing problem into two equal size sub-problems of size n/2.
  • We are performing 2 comparison operations in the combine step.
  • Base case situations occur when n = 2 or n = 1. When n = 2, we are performing 1 comparison operation and when n = 1, we are performing 0 comparison operation.

T(n) = T(n/2) + T(n/2) + 2 = 2T(n/2) + 2, where T(2) = 1 and T(1) = 0.

We can solve this recurrence relation using the recursion tree method or the master theorem. You can explore this blog post: How to analyze recursive functions? Here, we will use the recursion tree method to get the correct insight into the total comparison count. For a better understanding, let's assume that n is a power of 2.

After every level of recursion, the input size of the subproblems decreases by a factor of 1/2. So, the recursion will stop when the input size of the subproblems becomes 2 (base case). Let's suppose that after i number of levels, the input size reaches its base case.

=> 2 = n/2^i 
=> 2^(i+1) = n
Taking log both sides 

=> i + 1 = logn 
=> i = logn - 1
So the height of recursion tree = logn - 1

time complexity analysis of divide and conquer approach to find the minimum and maximum element

Note: Till (i - 1)th level, every subproblem will perform 2 comparisons at the combine step. The last level is the situation of base case, where only one comparison will be made.

  • Total comparison count from 0 to (i - 1) level = 2 + 4 + 8 + ... + 2^i = 2 [ 1 + 2 + 4 + ... + 2^(i - 1)] = 2*(2^i - 1). We get this value from the sum of geometric series.
  • Total comparison count at ith level = 2^i.
  • Total count of comparison operations = Total comparison count from 0 to (i - 1) level + Total comparison count at ith level = 2*(2^i - 1) + 2^i = 2^(i + 1) + 2^i - 2 = n + n/2 – 2 = 3n/2 – 2. Note: Here 2^(i + 1) = n and 2^i = n from the above analysis.

If n is not a power of 2, it will make more than 3n/2 - 2 comparisons. Overall time complexity = O(n). Here, time complexity is also O(n), but the total count of comparison operation is less than the previous approach.

Space complexity = The size of recursion call stack = The height of recursion tree = O(logn).

Efficient approach  using single loop: Increment by 2

Solution idea

In the first approach, we perform two comparison operations for every element in the worst case. Now the critical question is: can we optimize it further and reduce the total count of comparison operations?

One idea is to pick elements in pairs and try to update the minimum and maximum. Suppose we have updated the maximum and minimum in the max and min variables till the (i-1)th index. In the next iteration, we consider a pair of the ith and (i + 1)th indices.

  • If (X[i] < X[i + 1]): There can be a chance that X[i] is less than min and X[i + 1] is greater than max. In other words, X[i] will be the candidate for the minimum, and X[i + 1] will be the candidate for the maximum. So we compare X[i] with 'min' and X[i + 1] with 'max' to update the minimum and maximum till that point, i.e., if (X[i] < min), min = X[i] and if (X[i + 1] > max), max = X[i + 1].
  • If (X[i] > X[i + 1]): We can apply the same approach in this scenario. Here X[i + 1] will be the candidate for the minimum, and X[i] will be the candidate for the maximum, i.e., if (X[i] > max), max = X[i] and if (X[i + 1] < min), min = X[i + 1].

In both scenarios, we make three comparisons (in the worst case) to update the maximum and minimum of two elements. In other words, we save one comparison compared to the first approach where we need four comparisons for two elements (in the worst case).

Solution steps

Step 1: Declare the max and min variables and check for the array size. If array size is odd, we initialize the first element as both min and max. Otherwise, we compare the first two elements and set min to the smaller value and max to the larger value.

if (n % 2 != 0)
{
    max = X[0]
    min = X[0]
    i = 1
}
else
{
    if (X[0] < X[1])
    {
        max = X[1]
        min = X[0]
    }
    else
    {
        max = X[0]
        min = X[1]
    }
    i = 2
}

Step 2: Now we traverse the array and pick elements in pairs. For each pair (i, i + 1), compare both elements. Based on the comparison:

  1. We update the max by comparing the larger element with it.
  2. We update the min by comparing the smaller element with it.
while (i < n)
{
    if (X[i] < X[i + 1])
    {
        if (X[i] < min)
            min = X[i]
        if (X[i + 1] > max)
            max = X[i + 1]
    }
    else
    {
        if (X[i] > max)
            max = X[i]
        if (X[i + 1] < min)
            min = X[i + 1] 
    }
    i = i + 2
}

Step 3: Finally, we store max and min in an extra memory maxMin[2] and return it.

Solution pseudocode

int[] findMinMax(int X[], int n)
{
    int max, min, i
    if (n % 2 != 0)
    {
        max = X[0]
        min = X[0]
        i = 1
    }
    else
    {
        if (X[0] < X[1])
        {
            max = X[1]
            min = X[0]
        }
        else
        {
            max = X[0]
            min = X[1]
        }
        i = 2
    }
    while (i < n)
    {
        if (X[i] < X[i + 1])
        {
            if (X[i] < min)
                min = X[i]
            if (X[i + 1] > max)
                max = X[i + 1]
        }
        else
        {
            if (X[i] > max)
                max = X[i]
            if (X[i + 1] < min)
                min = X[i + 1] 
        }
        i = i + 2
    }
    int maxMin[2] = {max, min}
    return maxMin
}

Python implementation

def findMinMax(X, n):
    max, min, i = 0, 0, 0
    if n % 2 != 0:
        max = X[0]
        min = X[0]
        i = 1
    else:
        if X[0] < X[1]:
            max = X[1]
            min = X[0]
        else:
            max = X[0]
            min = X[1]
        i = 2
    while i < n:
        if X[i] < X[i+1]:
            if X[i] < min:
                min = X[i]
            if X[i+1] > max:
                max = X[i+1]
        else:
            if X[i] > max:
                max = X[i]
            if X[i+1] < min:
                min = X[i+1] 
        i = i + 2
    maxMin = [max, min]
    return maxMin

Solution analysis

For each pair, we perform three comparisons: first between the elements of the pair, and the other two with min and max. The total number of comparisons is 3 * (n-1) / 2 (if n is odd) or 3n/2 – 2 (if n is even). So, the time complexity is O(n). We are using constant extra space, so the space complexity is O(1).

We observe that the total number of comparisons is less than in the first approach. In other words, comparing in pairs helps us optimize the first approach further.

Critical ideas to think!

  • Why does incrementing the loop by two help reduce the comparison count?
  • How do we modify the above solutions when there are repeated input values?
  • Is there any other way to solve this problem?
  • In which scenario is the number of comparisons equal for approaches 2 and 3?
  • Why is the space complexity O(logn) in the divide and conquer solution? Why are there two base cases? What would be the time complexity if we remove the base case with an array size of 2?
  • What would be the best and worst case in the brute force approach?

Comparisons of time and space complexities

  • Brute force approach using a single loop: Time complexity = O(n), Space complexity = O(1), Total comparison count in the worst case = 2(n-1).
  • Using divide and conquer: Time complexity = O(n), Space complexity = O(logn), Total comparison count = 3n/2 - 2 (If n is a power of 2).
  • Efficient approach using a single loop: Time complexity = O(n), Space complexity = O(1), Total comparison count in the worst case = 3n/2 - 2.

Similar coding questions to practice

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