core pattern

Two Pointers

Use two pointers to traverse an array from different positions, often moving toward each other or in the same direction. Perfect for problems involving pairs, sorted arrays, or in-place modifications.

Time

O(n)

Space

O(1)

🧠Mental Model

“Squeezing a toothpaste tube from both ends — you converge toward the middle, processing as you go.”

Verbal cue: Start at edges, move inward based on comparison

🎯Recognition Triggers

When you see these patterns in a problem, consider this approach:

Sorted arrayFind pair with target sumRemove duplicates in-placePalindrome checkContainer with most waterThree sum / Four sumMerge sorted arrays

📝Code Template

Two Pointers Template

1function twoPointers(arr, target) {
2  let left = 0;
3  let right = arr.length - 1;
4  
5  while (left < right) {
6    const sum = arr[left] + arr[right];
7    
8    if (sum === target) {
9      return [left, right];
10    } else if (sum < target) {
11      left++;  // Need larger sum
12    } else {
13      right--; // Need smaller sum
14    }
15  }
16  
17  return [-1, -1]; // Not found
18}

Complexity rationale: Single pass through array, no extra space needed

💡Interview Tips

1Always clarify if the input is sorted — this determines the approach
2For "find all pairs" problems, continue searching after finding one
3Mention O(1) space as a key advantage over hash map approaches
4Draw the array and show pointer movement to visualize

⚠️Common Mistakes

✕Forgetting the array must be sorted for convergent two pointers
✕Off-by-one errors with left < right vs left <= right
✕Not handling duplicates when required (e.g., 3Sum)
✕Modifying pointers incorrectly after finding a match