Array unique and flip in PHP - Time & Space Complexity

Time Complexity: Array unique and flip

O(n)

Understanding Time Complexity

We want to understand how the time needed changes when we use PHP functions to remove duplicates and flip arrays.

How does the work grow as the array gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$array = [1, 2, 2, 3, 4, 4, 5];
$unique = array_unique($array);
$flipped = array_flip($unique);
print_r($flipped);
    

This code removes duplicate values from an array, then swaps keys and values in the resulting array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Traversing the array twice -- once for removing duplicates, once for flipping.
• How many times: Each function loops through the array once, so two passes total.

How Execution Grows With Input

As the array size grows, the number of steps grows roughly twice as fast because of two passes.

Input Size (n)	Approx. Operations
10	About 20 steps (2 x 10)
100	About 200 steps (2 x 100)
1000	About 2000 steps (2 x 1000)

Pattern observation: The work grows linearly with the input size, doubling because of two separate loops.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows directly in proportion to the number of items in the array.

Common Mistake

[X] Wrong: "Using array_unique and array_flip together makes the code run in quadratic time because of nested loops."

[OK] Correct: Each function loops through the array separately, not inside each other, so the total work adds up linearly, not multiplies.

Interview Connect

Understanding how built-in functions work under the hood helps you explain your code clearly and shows you know how performance scales with data size.

Self-Check

"What if we replaced array_unique with a manual loop that checks duplicates? How would the time complexity change?"