Two Pointer

Welcome to the Two Pointer section of JDCodebase! The Two Pointer technique is a powerful strategy to reduce nested loops by using two indices that move through the data structure. It's widely used for solving array and string problems efficiently.

What You’ll Learn

When and how to apply the two pointer technique
Solving sorted array problems with opposite-direction pointers
Detecting pairs/triplets with specific conditions
Avoiding unnecessary iterations and improving performance
Handling edge cases in left/right movement logic

Common Two Pointer Patterns

Opposite Ends: Start one pointer from the start and one from the end.

Before: [1, 2, 3, 4, 6]
Code: let left = 0, right = arr.length - 1;
  while (left < right) {
    let sum = arr[left] + arr[right];
    if (sum === target) return [left, right];
    sum > target ? right-- : left++;
  }
After: [1, 4] (example indices)

Same Direction: Both pointers start from the beginning or end and move in one direction.

Before: [1, 1, 2, 2, 3, 3]
Code: let i = 0, j = 1;
  while (j < arr.length) {
    if (arr[i] !== arr[j]) {
      i++;
      arr[i] = arr[j];
    }
    j++;
  }
After: Removes duplicates in-place

Try This Example

Check if a sorted array has two numbers that sum to a target.

function hasPairWithSum(arr, target) {
    let left = 0;
    let right = arr.length - 1;
  
    while (left < right) {
      const sum = arr[left] + arr[right];
      if (sum === target) return true;
      sum < target ? left++ : right--;
    }
  
    return false;
  }

Input: arr = [1, 2, 4, 4], target = 8

Expected Output: true