Detect Duplicates in a PHP Array with an O(n) Solution

Detecting duplicate elements in an array requires traversing each element and checking for repeated occurrences.

Introduction

Given an integer array nums, the function should return true if any value appears at least twice; otherwise it returns false.

Example:

Input: nums = [1,2,3,1]
Output: true

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

Explore Code

Below is a PHP class Solution with the method containsDuplicate($nums) that implements the detection:

class Solution {

    /**
     * @param Integer[] $nums
     * @return Boolean
     */
    function containsDuplicate($nums) {
        $map = array();
        foreach ($nums as $n => $i) {
            if (array_key_exists($i, $map)) {
                return true;
            }
            $map[$i] = $n;
        }
        return false;
    }
}

Function Execution Flow

1. Initialization: an empty associative array $map is created to store encountered elements and their indices.

2. Duplicate detection:

The function iterates over each element in $nums.

For each element, it checks whether the element already exists in $map.

If it exists, the function immediately returns true, indicating a duplicate.

If not, the element and its index are added to $map for future checks.

3. Return value: after scanning the entire array without finding duplicates, the function returns false, meaning the array has all unique elements.

Time and Space Complexity

Time complexity: O(n), where n is the number of elements in the input array, because each element is examined once.

Space complexity: O(n), as the associative array $map may store up to n unique elements in the worst case.

Conclusion

The optimized PHP solution uses an associative array for constant‑time lookups, enabling fast and reliable detection of duplicate elements while maintaining linear time and space performance.