Preploop
Patterns
DSA Patterns
Two PointersSDE I
Two indices scanning toward each other or at different speeds
O(n) O(1)
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Problem

Given a sorted array, find two numbers that add up to the target and return their indices.

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Controls
How to recognize this pattern
  • Problem gives a SORTED array and asks for a pair/triplet summing to X
  • Brute force is O(n²) nested loops — you need O(n)
  • Keywords: "two sum", "three sum", "palindrome", "remove duplicates in-place"
  • You can shrink the problem from both ends simultaneously
When to use this pattern

Instead of nested loops O(n²), use two indices that together cover the search space in a single pass. The sorted order lets you decide which pointer to move.

  • Sorted array and you need pairs/triplets that meet a condition
  • Partitioning: move all negatives left, positives right
  • Palindrome check, removing duplicates in-place
  • Fast + slow pointer: cycle detection, middle of linked list
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Speed
Input
Code Preview
O(n)
O(1)
1▶
function twoSum(arr, target) {
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  let left = 0, right = arr.length - 1;
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  while (left < right) {
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    const sum = arr[left] + arr[right];
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    if (sum === target) return [left, right];
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    else if (sum < target) left++;
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    else right--;
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  }
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  return [-1, -1];
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}