Cheatsheet›Two Pointers

CORE PATTERNHigh Frequency

Two Pointers

Use two indices that traverse a data structure — either toward each other, in the same direction, or across two separate arrays — to solve problems in linear time without extra space.

TimeO(N)

SpaceO(1)

DifficultyMedium

Key Insight

Two indices moving with purpose, not brute force.

Flip ⟲

Two Pointerstarget = 13

Sum17

MoveR−−

FLIP SIDETwo Pointers

Tap to flip back ⟲

RECOGNITION SIGNALS (WHEN TO USE THIS PATTERN)

✓Sorted array with pair/triplet target sum

✓Palindrome or symmetry check

✓Cycle detection in linked list

✓Contiguous subarray or substring with constraint

✓In-place rearrangement without extra space

✓Two sorted arrays to merge or compare

Types of Problems (Subpatterns)

Each subpattern solves a specific type of problem. Click on a card to view its template.

✓ SELECTED

Opposite Ends

L and R start at both ends and walk toward each other.

Two Sum II
3Sum
Container With Most Water

5 problems

Fast & Slow (Floyd's)

Slow moves 1 step, fast moves 2 — they meet only inside a cycle.

Linked List Cycle
Linked List Cycle II
Middle of the Linked List

5 problems

Partition (Slow / Fast Write)

Slow is the write head; fast scans and places valid elements.

Remove Duplicates from Sorted Array
Move Zeroes
Sort Colors (Dutch National Flag)

5 problems

Merge Two Arrays

One pointer per sorted array — pick the smaller element each step.

Merge Sorted Array
Intersection of Two Arrays II
Merge Two Sorted Lists

5 problems

Template / Approach

Template for: Opposite Ends

Approach

Place Pointers

Set L = 0 and R = n - 1 at opposite ends of the sorted array.

Evaluate Condition

Check the condition on arr[L] and arr[R] (e.g. their sum vs target).

Move Right Pointer

If the combined value is too large, move R left to decrease it.

Move Left Pointer

If the combined value is too small, move L right to increase it.

Stop When Crossed

Loop exits when L >= R — all pairs have been considered.

Template

</>Common Template

💡Universal opposite-ends skeleton. Works for any sorted-array pair/triplet problem. Replace the comparison and return logic with your problem's goal.

🔑

Key Idea:Sorted order guarantees: moving L right increases the sum, moving R left decreases it — so each step eliminates one possibility in O(1).

1// Opposite-ends two-pointer skeleton
2void oppositeEnds(vector<int>& arr)
3{
4    int L = 0;
5    int R = (int)arr.size() - 1;
6 
7    while (L < R)
8    {
9        int val = COMBINE(arr[L], arr[R]);   // e.g. arr[L] + arr[R]
10 
11        if (val == TARGET)
12        {
13            // ── found answer ─────────────────────────
14            RECORD(L, R);
15            L++;
16            R--;
17        }
18        else if (val < TARGET)
19        {
20            L++;   // need a larger value
21        }
22        else
23        {
24            R--;   // need a smaller value
25        }
26    }
27}

5 int R = (int)arr.size() - 1;

7 while (L < R)

8 {

9 int val = COMBINE(arr[L], arr[R]); // e.g. arr[L] + arr[R]

11 if (val == TARGET)

12 {

13 // ── found answer ─────────────────────────

14 RECORD(L, R);

15 L++;

16 R--;

17 }

18 else if (val < TARGET)

19 {

20 L++; // need a larger value

21 }

22 else

23 {

24 R--; // need a smaller value

25 }

26 }

27}

Two Pointers

RECOGNITION SIGNALS (WHEN TO USE THIS PATTERN)

Types of Problems (Subpatterns)

Opposite Ends

Fast & Slow (Floyd's)

Partition (Slow / Fast Write)

Merge Two Arrays

Template / Approach

Approach

Place Pointers

Evaluate Condition

Move Right Pointer

Move Left Pointer

Stop When Crossed

Template

Practice Problems

Two Sum II - Input Array Is Sorted

3Sum

Container With Most Water

Valid Palindrome

Trapping Rain Water

4Sum

Linked List Cycle

Linked List Cycle II

Middle of the Linked List

Happy Number

Palindrome Linked List

Remove Duplicates from Sorted Array

Move Zeroes

Sort Colors

Remove Element

Remove Duplicates from Sorted Array II

Merge Sorted Array

Intersection of Two Arrays II

Backspace String Compare

Median of Two Sorted Arrays

Two Pointers

RECOGNITION SIGNALS (WHEN TO USE THIS PATTERN)

Types of Problems (Subpatterns)

Opposite Ends

Fast & Slow (Floyd's)

Partition (Slow / Fast Write)

Merge Two Arrays

Template / Approach

Approach

Place Pointers

Evaluate Condition

Move Right Pointer

Move Left Pointer

Stop When Crossed

Template

Practice Problems

Two Sum II - Input Array Is Sorted

3Sum

Container With Most Water

Valid Palindrome

Trapping Rain Water

4Sum

Linked List Cycle

Linked List Cycle II

Middle of the Linked List

Happy Number

Palindrome Linked List

Remove Duplicates from Sorted Array

Move Zeroes

Sort Colors

Remove Element

Remove Duplicates from Sorted Array II

Merge Sorted Array

Intersection of Two Arrays II

Backspace String Compare

Median of Two Sorted Arrays