Two Pointers Pattern

When to use Two Pointers

• The input is sorted (or you are allowed to sort it first) and you need a pair, triplet, or k-tuple that satisfies a condition.
• You are searching for a subarray or substring where the answer depends on both endpoints rather than a single index.
• You need to compare characters from both ends of a string, e.g. palindrome checks.
• You can reduce a brute-force O(n^2) nested loop on a one-dimensional input to a single pass.
• You need to partition an array in place (Dutch national flag, remove duplicates, move zeros).

Template

function twoPointers(nums, target) {
  let left = 0;
  let right = nums.length - 1;
  while (left < right) {
    const sum = nums[left] + nums[right];
    if (sum === target) {
      return [left, right];
    } else if (sum < target) {
      left++;
    } else {
      right--;
    }
  }
  return [-1, -1];
}

Common mistakes

• Applying Two Pointers to an unsorted array without first sorting (or recognizing that sorting would change the required output order).
• Forgetting to skip duplicates after moving a pointer in problems like 3Sum, which produces repeated tuples in the result.
• Using `<=` instead of `<` in the loop condition, which double-counts the same index in pair-sum problems.
• Moving the wrong pointer when the sum overshoots or undershoots the target.
• Mutating the array in place when the problem requires the original order to be preserved for later checks.