Two Pointers

Subtitle / Summary The hard part is not deletion itself, but locating the predecessor of the nth node from the end in a singly linked list. This article derives the one-pass two-pointer solution from simpler baselines and explains correctness, boundaries, and engineering transfer. Reading time: 12-15 min Tags: linked list, two pointers, interview high frequency SEO keywords: LeetCode 19, Remove Nth Node From End of List, linked list, fast/slow pointers, dummy node Meta description: A complete ACERS walkthrough for removing the nth node from the end: from brute force to one-pass two pointers, with complexity, pitfalls, engineering scenarios, and Python/C/C++/Go/Rust/JS implementations. Target Readers Beginners building a stable template for linked-list interview problems Developers who know fast/slow pointers but still make boundary mistakes Backend/system engineers who want to transfer problem-solving templates to chain-structured data in production Background / Motivation “Remove the nth node from the end” is a classic medium-level linked-list problem. The challenge is usually not the delete operation itself, but: ...

Subtitle / Summary LeetCode 142 upgrades cycle detection into cycle entry localization. The robust template is Floyd: first detect a meeting inside the cycle, then reset one pointer to head and move both by one step; the next meeting node is the cycle entry. Reading time: 12-16 min Tags: Hot100, linked list, fast slow pointers, Floyd SEO keywords: Linked List Cycle II, cycle entry, Floyd, fast slow pointers, O(1) space, LeetCode 142, Hot100 Meta description: Floyd cycle detection + entry localization with proof intuition, engineering mapping, and runnable multi-language implementations in O(n) time and O(1) extra space. Target Readers Hot100 learners who want to fully internalize the 141 -> 142 linked-list template family Developers who need to locate where a pointer chain becomes cyclic Interview candidates who want to explain why “reset to head” works Background / Motivation In real systems, cycle corruption in chain structures can cause: ...

Subtitle / Summary This problem is the linked-list version of merge-sort’s merge step. Use a sentinel node plus two pointers to splice nodes in ascending order in O(m+n), without rebuilding the list. Reading time: 10-12 min Tags: Hot100, linked list, merge, two pointers SEO keywords: Merge Two Sorted Lists, sentinel node, linked list merge, LeetCode 21, Hot100 Meta description: A complete ACERS guide for LeetCode 21 with derivation, correctness invariants, pitfalls, and runnable multi-language code. Target Readers Hot100 learners preparing linked-list interview templates Developers who often lose nodes while rewiring next Engineers who need stable O(1)-extra-space merge patterns Background / Motivation This is a small problem with large transfer value: ...

Subtitle / Summary LeetCode 25 is the upgrade path from 206 (full reversal) and 92 (interval reversal): split by groups, reverse inside each full group, reconnect safely, and keep the last incomplete group unchanged. Reading time: 14-18 min Tags: Hot100, linked list, group reversal, dummy node SEO keywords: Reverse Nodes in k-Group, group reversal, LeetCode 25, Hot100 Meta description: In-place k-group linked-list reversal with dummy-node anchoring and safe reconnection, including pitfalls, complexity, and runnable multi-language implementations. Target Readers Hot100 learners who already finished 206/92 and want the next linked-list jump Developers who often fail at boundary handling and reconnection in pointer-heavy tasks Engineers building reusable templates for chunk-based list transformation Background / Motivation In real systems, chain structures are often processed in batches: ...

Subtitle / Summary Reorder List is a classic pointer choreography problem: find middle, reverse second half, then merge alternately. This guide derives the in-place O(n)/O(1) method from naive ideas and turns it into a reusable Hot100 template. Reading time: 12-15 min Tags: Hot100, linked list, in-place SEO keywords: Reorder List, split reverse merge, LeetCode 143, O(1) space Meta description: A full ACERS explanation of Reorder List with correctness intuition, boundary handling, engineering mapping, and runnable code in Python/C/C++/Go/Rust/JS. Target Readers Hot100 learners who want a stable linked-list rewire template Developers who can reverse lists but still fail on alternating merge details Engineers preparing for interviews where O(1) extra space is required Background / Motivation At first glance, this looks like simple reordering. In reality, it tests whether you can safely perform three dependent pointer operations in one workflow: ...

Subtitle / Summary Detecting a cycle in a linked list is a pointer chasing problem, not a value comparison problem. This ACERS guide explains why Floyd’s fast/slow pointers must meet if a cycle exists, how to avoid null-pointer bugs, and how the same pattern maps to engineering checks. Reading time: 10-12 min Tags: Hot100, linked list, fast slow pointers, Floyd SEO keywords: Linked List Cycle, Floyd, fast slow pointers, LeetCode 141, Hot100 Meta description: O(n)/O(1) cycle detection in singly linked lists using Floyd fast/slow pointers, with alternatives, common mistakes, and runnable multi-language code. Target Readers Hot100 learners and interview candidates Developers building reusable linked-list two-pointer templates Engineers who need to detect loops in chain-like structures Background / Motivation Cycle bugs are common in pointer-linked structures: ...

Subtitle / Summary The key is not comparing values, but comparing node identity (same object / same address). This ACERS guide explains the naive hash approach, the length-alignment approach, and the most practical switch-head two-pointer template, with runnable multi-language implementations under the no-modification and no-cycle constraints. Reading time: 10-14 min Tags: Hot100, linked list, two pointers SEO keywords: Intersection of Two Linked Lists, switch heads, O(1) space, LeetCode 160 Meta description: Two pointers walk A then B and B then A, guaranteeing meeting at the intersection or both reaching null within m+n steps, with O(m+n) time and O(1) space. Target Readers Hot100 learners who want a reusable linked-list two-pointer template Developers who often confuse “same value” with “same node” Engineers working with shared tail structures in chain-like data Background / Motivation This problem looks simple, but it forces you to separate three concepts: ...

Subtitle / Summary Trapping Rain Water is the classic boundary-constraint problem. This ACERS guide explains the two-pointer method, key formulas, and runnable multi-language solutions. Reading time: 12–15 min Tags: Hot100, two pointers, array SEO keywords: Trapping Rain Water, two pointers, left right max, O(n), Hot100 Meta description: Two-pointer O(n) trapped water solution with engineering scenarios and multi-language code. Target Readers Hot100 learners building core templates Engineers handling capacity/volume constraints Anyone who wants a clean O(n) solution Background / Motivation Trapped water is a proxy for “capacity under boundary constraints.” It appears in cache headroom estimation, buffer overflow analysis, and terrain capacity modeling. The naive O(n^2) method is too slow; the two-pointer approach reduces it to O(n). ...

Subtitle / Summary A classic event-spacing validation model. This ACERS guide explains the one-pass logic, engineering use cases, and runnable multi-language solutions. Reading time: 10–12 min Tags: array, two pointers, event spacing SEO keywords: LeetCode 1437, event spacing, O(n) Meta description: One-pass validation for minimum spacing between 1s, with engineering use cases and multi-language code. Target Readers LeetCode learners building stable templates Engineers working on monitoring / risk control / behavior analytics Developers who need spacing or rate-limit validations Background / Motivation Many systems require events to be spaced apart: login failures, alarms, sensitive actions, API calls, etc. This problem maps directly to event spacing validation. A one-pass, O(1)-memory solution is ideal for real-time systems. ...