Two Pointers Pattern

The Two Pointers technique uses two pointers to traverse a data structure, often from opposite ends or at different speeds.

When to Use

• Working with sorted arrays
• Finding pairs that satisfy a condition
• In-place array modifications
• Reducing O(n²) to O(n)

Interactive Visualization

This example finds two numbers that sum to a target value:

1function twoSum(arr, target) {
2  // Array must be sorted
3  let left = 0;
4  let right = arr.length - 1;
5 
6  while (left < right) {
7    const sum = arr[left] + arr[right];
8 
9    if (sum === target) {
10      return [left, right];
11    } else if (sum < target) {
12      left++;
13    } else {
14      right--;
15    }
16  }
17  return null; // No pair found
18}

How It Works

1. Initialize two pointers (usually start and end)
2. Calculate current result based on pointer positions
3. Move pointers based on comparison with target:
   • Too small? Move left pointer right
   • Too big? Move right pointer left
   • Found? Return result

Complexity

Common Variations

Opposite Ends

Start from both ends, move towards center.

Same Direction

Both pointers start from same end, move at different speeds.

Slow & Fast Pointer

One moves 1 step, other moves 2 steps (cycle detection).

Practice Problems

1. Container With Most Water
2. 3Sum
3. Remove Duplicates from Sorted Array
4. Valid Palindrome