Two Pointers & Sliding Window

Quick reference for when to use each pattern. Full content: Arrays, Strings.

Two Pointers

When: Pairs, triplets (fix one + two pointers), validity from both ends.
Examples: Two Sum (if sorted), 3Sum, Container With Most Water, Valid Palindrome.
Template: left=0, right=n-1; move left or right based on comparison; O(N) or O(N²) with fix.

When: Partition (move zeros), remove duplicates in-place, or "fast/slow" on linked list (see Linked List).
Examples: Move Zeroes, Remove Duplicates from Sorted Array, Linked List cycle/middle.

When: "Longest/shortest contiguous subarray/substring" with constraint (sum, at most K distinct, etc.).
Template: Expand j, then shrink i until valid; update answer. O(N).
Examples: Longest Substring Without Repeating Characters, Minimum Window Substring, Longest Substring with At Most K Distinct Characters.

When: "Max/min in every window of size K" or "all subarrays of size K".
Examples: Max in sliding window → monotonic deque; or simple scan per window O(N*K) → O(N) with deque.
See: Queue for monotonic deque implementation.

Clue

Pattern

Sorted array + pair/triplet

Two pointers (opposite ends)

Contiguous + "longest/shortest" + condition

Sliding window (variable)

"Max/min in each window of size K"

Monotonic deque (or two pointers)

In-place partition / move

Two pointers (same direction)

"At most K" distinct/sum

Sliding window + map/count

Two pointers opposite: left/right; move based on value vs target; never miss in sorted array.
Sliding window: [i,j); add j, then while invalid remove i; candidate = j-i+1.
Edge cases: Empty; K=0; all same; negative numbers (window sum).

Last updated 12 days ago