Split the Array

Split the Array

What is this problem about?

The Split the Array coding problem asks if it's possible to take an array of integers and divide all its elements into two separate arrays of equal length, such that each of the two arrays contains only unique elements. The total number of elements in the original array is always even. Effectively, you need to check if you can distribute the numbers such that no number appears more than twice in the original array, and if it appears twice, one goes to the first array and one to the second.

Why is this asked in interviews?

This "Easy" problem is used by Google and Adobe to test basic counting and frequency analysis. It's a great way to see if a candidate can translate a wordy problem into a simple frequency constraint. The core logic boils down to: "Can any number appear more than twice?" If the answer is no, then a valid split is always possible.

Algorithmic pattern used

The pattern used is Hash Table (or Frequency Map) and Counting. You iterate through the array and count the frequency of each element. If any element's frequency exceeds 2, it's impossible to put those elements into two arrays while keeping both arrays unique (since at least one array would end up with two of the same element). If every element appears either once or twice, you can easily distribute them.

Example explanation (use your own example)

Suppose you have an array [1, 1, 2, 2, 3, 4].

• 1 appears twice.
• 2 appears twice.
• 3 appears once.
• 4 appears once. Since no number appears more than twice, we can split them: Array 1: [1, 2, 3] Array 2: [1, 2, 4] Both arrays are the same length (3) and have unique elements. The answer is true. However, if the array was [1, 1, 1, 2, 3, 4], 1 appears three times. No matter how you split them into two arrays, one array will get at least two 1s. Thus, the answer would be false.

Common mistakes candidates make

• Overcomplicating the split: Trying to actually build the two arrays when only a boolean "possible or not" is required.
• Ignoring the "equal length" constraint: Although for this specific problem, if frequency is $\le 2$, an equal split is always mathematically guaranteed, some candidates get distracted by trying to balance the unique elements.
• Frequency map errors: Using an inefficient way to count occurrences.
• Misreading the unique constraint: Thinking only one of the arrays needs unique elements, rather than both.

Interview preparation tip

For Array, Hash Table, Counting interview pattern problems, always consider if a frequency map can simplify the problem. Most "distribution" or "grouping" problems rely heavily on understanding the counts of individual elements.