Mastering the Two Pointers Technique

The two pointers technique is a fundamental approach to solving array and string problems efficiently. This pattern involves using two pointers that either move toward each other or in the same direction to solve the problem with optimal time complexity.

Common Use Cases

1. Finding Pairs in a Sorted Array

   • Finding pairs that sum to a target value
   • Finding triplets that sum to zero
   • Container with most water

2. String Manipulation

   • Palindrome verification
   • Reversing strings
   • Removing duplicates

Example Problem: Valid Palindrome

Let's solve a common interview problem using the two pointers technique:

function isPalindrome(s: string): boolean {
    // Convert to lowercase and remove non-alphanumeric characters
    s = s.toLowerCase().replace(/[^a-z0-9]/g, '');
    
    let left = 0;
    let right = s.length - 1;
    
    while (left < right) {
        if (s[left] !== s[right]) {
            return false;
        }
        left++;
        right--;
    }
    
    return true;
}

Time Complexity Analysis

• Time Complexity: O(n)
• Space Complexity: O(1)

Best Practices

1. Always validate inputs

   • Check for empty arrays/strings
   • Handle edge cases

2. Choose the right pointer movement

   • Towards each other (like in palindrome checking)
   • Same direction (like in slow/fast pointer problems)

3. Consider sorting

   • Some two pointer problems require sorted arrays
   • Factor in the sorting time complexity

Common Pitfalls

1. Boundary Conditions

   • Off-by-one errors
   • Handling equal pointers

2. Pointer Movement

   • Infinite loops
   • Skipping elements

Practice Problems

1. Container With Most Water (LeetCode #11)
2. 3Sum (LeetCode #15)
3. Remove Duplicates from Sorted Array (LeetCode #26)

Key Takeaways

• Two pointers is a powerful technique for optimizing array/string problems
• Often reduces time complexity from O(n²) to O(n)
• Particularly useful for sorted arrays and palindrome problems
• Remember to handle edge cases and boundary conditions