How is two pointers different from a sliding window?

Two pointers reads the values at each index. A window tracks everything between them, such as a sum or a count.

Does the array have to be sorted?

For opposite-end pointers, yes. Same-direction pointers, like removing duplicates in place, work on any order the problem gives you.

Pattern · 20 problems

Two Pointers Interview Problems

Two indexes beat a nested loop when the input has order or symmetry. Every move has to rule something out for good.

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When I reach for it

The array is sorted, or you can sort it.
You need a pair, a triple or a palindrome.
You shrink from both ends, or one index chases another.

Problems, easiest first

5 easy · 13 medium · 2 hard. Open one to read it, then write it in the free editor.

Linked List CycleeasyLinked Lists
Two SumeasyArrays & Hashing
Valid PalindromeeasyStrings
Happy NumbereasyArrays & Hashing
Middle of the Linked ListeasyLinked Lists
3SummediumArrays & Hashing
Longest Palindromic SubstringmediumDynamic Programming Strings
Longest Repeating Character ReplacementmediumSliding Window Strings
Meeting Rooms IImediumIntervals Greedy
Palindromic SubstringsmediumDynamic Programming Strings
Reorder ListmediumLinked Lists Stack
Container With Most WatermediumGreedy Arrays & Hashing
Remove Nth Node From End of ListmediumLinked Lists
Find the Duplicate NumbermediumBit Manipulation Binary Search
Partition LabelsmediumGreedy Strings
Permutation in StringmediumSliding Window Strings
Two Sum II - Input Array Is SortedmediumArrays & Hashing
Sort ColorsmediumArrays & Hashing
Find Median from Data StreamhardHeaps Design
Trapping Rain WaterhardStack Heaps

The idea

Each pointer move must throw away options that cannot win. If you cannot say what a move rules out, the pointers are wandering.

Opposite ends work when order tells you which side to move. Same-direction pointers work when one index reads and the other writes.

left, right = 0, len(nums) - 1
while left < right:
    total = nums[left] + nums[right]
    if total == target:
        return [left, right]
    if total < target:
        left += 1      # too small: only a bigger left can help
    else:
        right -= 1     # too big: only a smaller right can help

Compare the two ends, then move the side that cannot help. Order is what makes each move safe. Python shown; the idea carries to every language in the editor.

Where it goes wrong

Moving the wrong pointer. Say why the move is safe before you make it.
Forgetting that the array has to be sorted for opposite-end pointers.
Counting duplicates twice in three-sum style problems.

Say this before you type. Say what each pointer means and what a move to the left or right rules out.

Where Coco comes in

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Questions people ask

How is two pointers different from a sliding window?: Two pointers reads the values at each index. A window tracks everything between them, such as a sum or a count.
Does the array have to be sorted?: For opposite-end pointers, yes. Same-direction pointers, like removing duplicates in place, work on any order the problem gives you.

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