Two Pointers

The two-pointer technique walks two indices through a sequence — from both ends inward, or one ahead of the other — to solve a problem in a single pass and O(1) extra space. Reach for it on sorted arrays or strings when you're looking for a pair, a triplet, or a partition and a brute-force double loop would be O(n²).

24 LeetCode problems solved with the Two Pointers pattern. Practice them with spaced repetition so the pattern sticks.

• 3SumMEDIUM · O(n^2)
• 4SumMEDIUM · O(n^3)
• Concatenation of ArrayEASY · O(n)
• Container With Most WaterMEDIUM · O(n)
• Is SubsequenceEASY · O(n)
• Merge Sorted ArrayEASY · O(m + n)
• Merge Strings AlternatelyEASY · O(n + m)
• Move ZeroesEASY · O(n)
• Next PermutationMEDIUM · O(n)
• Remove Duplicates from Sorted ArrayEASY · O(n)
• Remove Duplicates from Sorted Array IIMEDIUM · O(n)
• Remove ElementEASY · O(n)
• Reverse StringEASY · O(n)
• Reverse Vowels of a StringEASY · O(n)
• Reverse Words in a StringMEDIUM · O(n)
• Rotate ArrayMEDIUM · O(n)
• Sort ColorsMEDIUM · O(n)
• Squares of a Sorted ArrayEASY · O(n)
• String CompressionMEDIUM · O(n)
• Summary RangesEASY · O(n)
• Trapping Rain WaterHARD · O(n)
• Two Sum II - Input Array Is SortedMEDIUM · O(n)
• Valid PalindromeEASY · O(n)
• Valid Palindrome IIEASY · O(n)