Two Pointers

5 problems using this pattern

About this pattern:

Use two pointers moving towards each other or in the same direction to solve in O(n) time.

Often used with: Array Binary Search Greedy String

Complexity Range

Time Complexity

O(log n) (1 problem)
O(n) (4 problems)

Space Complexity

O(1) (5 problems)

Problems Using Two Pointers

Find First and Last Position

Find the starting and ending position of a given target value.

Medium LeetCode
Time: O(log n)
Space: O(1)
Example: nums = [5,7,7,8,8,10], target = 8 → [3,4]
Binary Search Two Pointers
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Valid Palindrome

Determine if string is a palindrome, ignoring non-alphanumeric characters and case.

Easy LeetCode
Time: O(n)
Space: O(1)
Example: "A man, a plan, a canal: Panama" → true
Two Pointers String
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Two Sum II - Input Array Is Sorted

Find two numbers in sorted array that add up to target. Return 1-indexed positions.

Medium LeetCode
Time: O(n)
Space: O(1)
Example: [2,7,11,15], target=9 → [1,2]
Two Pointers Array
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Best Time to Buy and Sell Stock

Find maximum profit from buying and selling stock once.

Easy LeetCode
Time: O(n)
Space: O(1)
Example: [7,1,5,3,6,4] → 5 (buy at 1, sell at 6)
Two Pointers Greedy
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Trapping Rain Water

Calculate how much water can be trapped after raining given elevation heights.

Hard LeetCode
Time:
O(n)?
Why O(n)?
Single pass with two pointers moving towards center
We visit each element exactly once. Each position is processed when either left or right pointer reaches it.
Example: Array of 12 elements needs exactly 12 operations (6 from each pointer)
Space:
O(1)?
Why O(1)?
Only using a few variables regardless of input size
We only store left_max, right_max, and two pointers - constant space that doesn't grow with input.
Example: Same memory usage for array of 10 or 10,000 elements
Example: [0,1,0,2,1,0,1,3,2,1,2,1] → 6
Two Pointers Array
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