The two pointers pattern: how to spot it

Updated Oct 15, 2026

Two pointers walk the input from two places at once, usually the two ends of a sorted array or string, and each step rules out a whole set of answers. That is how an O(n²) pair search becomes O(n).

When to reach for it

• The input is sorted, or sorting it does not lose anything you need to return.
• You are looking for a pair (or a triple, by fixing one value) that meets a target.
• The question compares the two ends of something, like a palindrome or a container.
• Moving one end can only make the answer better or worse in a predictable direction.

left, right = 0, len(items) - 1
while left < right:
    total = items[left] + items[right]
    if total == target:
        return left, right
    if total < target:
        left += 1               # need bigger: drop the smallest
    else:
        right -= 1              # need smaller: drop the largest

4 LeetCode two pointers problems, easiest first

1. Valid Palindrome easy

   Skip junk, lowercase, then close the gap. Mismatch is the answer.

2. Two Sum II medium

   Sorted input: too small, advance left; too large, retreat right.

3. 3Sum medium

   Sort, fix i, then two-sum the suffix. Skip duplicates at every pointer.

4. Container With Most Water medium

   Area is limited by the shorter wall. Move that end; the other cannot help yet.

Skills these problems use

Each one has a free lesson with worked examples and a drill.

• Place opposite-end pointers
• Target-sum with opposite pointers
• Require sorted order before facing pointers
• Move left when the pair sum is too small
• Fix one index then face the rest
• Skip equal neighbors after a hit
• Close the gap from both ends
• Decide which end to advance

All two pointers lessons

Other patterns

• Hash map
• Hash set
• Array state
• Sliding window