LeetCode #1: Two Sum Problem Solution in PHP

📌 Introduction

The Two Sum problem is one of the most popular coding interview questions and is often asked in companies like Google, Amazon, and Microsoft.

In this blog, we will solve the problem using PHP, starting from a basic approach to an optimized solution.

🧠 Problem Statement

Given an array of integers nums and an integer target, return the indices of two numbers such that they add up to the target.

👉 Conditions:

• Each input has exactly one solution
• You cannot use the same element twice
• Return answer in any order

📥 Example

Input: nums = [2,7,11,15], target = 9  
Output: [0,1]

👉 Explanation:
nums[0] + nums[1] = 2 + 7 = 9

Input: nums = [2,7,11,15], target = 9  
Output: [0,1]

👉 Explanation:
nums[0] + nums[1] = 2 + 7 = 9

🧩 Approach 1: Brute Force (Simple Method)

🔍 Logic

Check every pair of elements and see if they add up to the target.

💻 PHP Code

function twoSum($nums, $target) {
    $n = count($nums);    
    for ($i = 0; $i < $n; $i++) {
        for ($j = $i + 1; $j < $n; $j++) {
            if ($nums[$i] + $nums[$j] == $target) {
                return [$i, $j];
            }
        }
    }
    return [];
}

function twoSum($nums, $target) {
    $n = count($nums);    
    for ($i = 0; $i < $n; $i++) {
        for ($j = $i + 1; $j < $n; $j++) {
            if ($nums[$i] + $nums[$j] == $target) {
                return [$i, $j];
            }
        }
    }
    return [];
}

⏱ Time Complexity

• O(n²) ❌ (Slow for large input)

⚡ Approach 2: Optimized (Hash Map Method)

🔍 Logic

• Store numbers in an array (map)
• Check if target - current already exists

👉 This avoids nested loops!

💻 PHP Code

function twoSum($nums, $target) {
    $map = [];    
    foreach ($nums as $index => $num) {
        $diff = $target - $num;        
        if (isset($map[$diff])) {
            return [$map[$diff], $index];
        }        $map[$num] = $index;
    }    
    return [];
}

function twoSum($nums, $target) {
    $map = [];    
    foreach ($nums as $index => $num) {
        $diff = $target - $num;        
        if (isset($map[$diff])) {
            return [$map[$diff], $index];
        }        $map[$num] = $index;
    }    
    return [];
}

⏱ Time Complexity

• O(n) ✅ (Efficient)

🔄 Dry Run

Input: [2,7,11,15], target = 9

🚀 Why This Question is Important

• 🧠 Tests problem-solving skills
• 🔑 Introduces hash maps
• ⚡ Teaches optimization
• 💼 Common in interviews

⚠️ Common Mistakes

• ❌ Using same index twice
• ❌ Not returning indices
• ❌ Forgetting optimized approach

🛠 Best Practices

• Always think of optimization
• Use associative arrays (maps) in PHP
• Practice dry runs

🎯 Conclusion

The Two Sum problem is a must-know for every developer.
Start with brute force, then move to optimized solutions using hash maps.