#2170

Medium

auto_awesomeArray scanning plus hash lookup

LeetCode Problem Workspace

Minimum Operations to Make the Array Alternating

Given an array, calculate the minimum number of operations needed to make it alternating.

Problem Statement

You are given an array of integers, and your task is to transform it into an alternating sequence. An alternating sequence is one where adjacent elements are different. You can perform one operation by changing the value of an element to any positive integer.

The goal is to find the minimum number of operations required to convert the given array into an alternating sequence. The array may not already be alternating, and each element can be changed any number of times.

Examples

Example 1

Input: nums = [3,1,3,2,4,3]

Output: 3

One way to make the array alternating is by converting it to [3,1,3,1,3,1]. The number of operations required in this case is 3. It can be proven that it is not possible to make the array alternating in less than 3 operations.

Example 2

Input: nums = [1,2,2,2,2]

Output: 2

One way to make the array alternating is by converting it to [1,2,1,2,1]. The number of operations required in this case is 2. Note that the array cannot be converted to [2,2,2,2,2] because in this case nums[0] == nums[1] which violates the conditions of an alternating array.

Constraints

1 <= nums.length <= 105
1 <= nums[i] <= 105

Solution Approach

Count Frequencies for Odd and Even Indexed Elements

First, split the array into two groups: one containing elements at even indices and the other containing elements at odd indices. Then, count the frequency of each element in these two groups separately. The most frequent elements will determine the minimum number of changes required.

Identify Optimal Element for Each Group

Find the most common element for both the odd-indexed and even-indexed groups. The key is to identify two most frequent elements, one from each group, such that they are not the same. If the most frequent elements in both groups are the same, swap them to minimize operations.

Minimize Changes by Swapping

Once the most frequent elements are found, calculate how many changes are needed to make the array alternating. If the most frequent element in the odd and even indices groups are the same, swap one of them to minimize the number of operations.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of the solution is O(n) for counting the frequencies and determining the most frequent elements. Space complexity is O(n) due to storing the frequency counts of each element in the odd and even indexed groups.

What Interviewers Usually Probe

This problem tests the ability to work with arrays and hash tables efficiently.
Check the candidate's ability to optimize solutions using counting techniques.
Look for a clear understanding of how to handle alternating sequence constraints.

Common Pitfalls or Variants

Common pitfalls

Failing to split the array into odd and even indexed groups properly.
Not considering swapping the most frequent elements when they overlap.
Misunderstanding the number of operations needed when both the odd and even indexed groups share common elements.

Follow-up variants

Using different methods for frequency counting.
Handling larger input sizes efficiently.
Considering edge cases like arrays of length 1 or 2.

FAQ

What is the minimum number of operations required to make an array alternating?

The minimum number of operations is determined by counting the frequency of elements at odd and even positions, then swapping elements if necessary.

How do I solve the Minimum Operations to Make the Array Alternating problem?

First, split the array into odd and even indexed groups. Count frequencies, identify the most frequent elements, and then minimize changes by swapping elements if necessary.

What is the primary pattern used in the Minimum Operations to Make the Array Alternating problem?

The primary pattern involves array scanning combined with hash lookup to count element frequencies and optimize the number of operations.

What are the constraints of the Minimum Operations to Make the Array Alternating problem?

The array length must be between 1 and 10^5, and each element in the array must be between 1 and 10^5.

What are common pitfalls when solving the Minimum Operations to Make the Array Alternating problem?

Common pitfalls include failing to handle element overlaps between odd and even indexed groups, not counting frequencies correctly, and misunderstanding the operations needed for alternating sequences.

terminal

Solution

Solution 1: Maintain Count of Odd and Even Positions

According to the problem description, if an array $\textit{nums}$ is an alternating array, then the elements at odd positions and even positions must be different, and the elements at odd positions are the same, as well as the elements at even positions.

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class Solution:
    def minimumOperations(self, nums: List[int]) -> int:
        def f(i: int) -> Tuple[int, int, int, int]:
            k1 = k2 = 0
            cnt = Counter(nums[i::2])
            for k, v in cnt.items():
                if cnt[k1] < v:
                    k2, k1 = k1, k
                elif cnt[k2] < v:
                    k2 = k
            return k1, cnt[k1], k2, cnt[k2]

        a, b = f(0), f(1)
        n = len(nums)
        if a[0] != b[0]:
            return n - (a[1] + b[1])
        return n - max(a[1] + b[3], a[3] + b[1])

Continue Practicing

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