Unify "search for an element," "search for the left boundary," and "search for the right boundary" into a single half-open interval skeleton [left, right): how to fill in the three blanks (condition, shrink direction, return value), why lowerBound and upperBound differ by just one comparison operator, why the right boundary is upperBound - 1, and three troubleshooting checklists for mid overflow, infinite loops, and out-of-bounds return values. Includes implementations in C, C++, and JavaScript.

LeetCode 370 Range Addition, 1109 Corporate Flight Bookings, and 1094 Car Pooling all reduce to the same "mark both endpoints + prefix-sum restore" template: how to write it with an extra slot, how to convert 1-indexed input, and why the end is right-open, with implementations in C, C++, and JavaScript. The post then extends to 2D difference arrays, tree difference, sweep-line thinking, and structure selection.

Converge LeetCode 303 and 304, both "Immutable" problems, onto a single "one extra slot" template: how 1D pre[r+1] - pre[l] and the 2D inclusion-exclusion four corners work, with implementations in C, C++, and JavaScript. Then look at four kinds of problems prefix sums can't handle (dynamic updates, operations without inverses, overflow, and lost internal structure) and what structures to use instead.

A deep dive into four advanced sliding window problems: Contains Duplicate II, Contains Duplicate III, Minimum Operations to Reduce X to Zero, and Longest Substring with At Least K Repeating Characters. Each demonstrates a different variation—swapping window state for hash maps and bucketing, reversing the problem, and enumerating dimensions—with C / C++ / JavaScript implementations.

A detailed walkthrough of four variable-length window problems: Minimum Size Subarray Sum, Subarray Product Less Than K, Max Consecutive Ones III, and Longest Repeating Character Replacement. Each problem includes answers to three key questions, step-by-step diagrams, common pitfalls, and implementations in C, C++, and JavaScript.

Distill a unified sliding window template from four classic problems—minimum window substring, permutation in string, find all anagrams in a string, and longest substring without repeating characters—by answering three questions: when to expand, when to shrink, and when to update the result. Also explains why the window interval is best written as left-closed, right-open. Includes implementations in C, C++, and JavaScript.

A detailed walkthrough of the last four problems in LeetCode's "Array and Two Pointers" introductory plan: sorting a matrix diagonally, shifting a 2D grid, transposing a matrix, and finding the longest common prefix. Covers techniques such as identifying diagonals with i - j, flattening with index mapping, triple reversal, and vertical scanning, with implementations in C, C++, and JavaScript.

A walkthrough of the first five problems in LeetCode's "Array and Two Pointers" introductory plan: removing duplicates from a sorted array, validating a palindrome, sorting colors, merging two sorted arrays, and squaring a sorted array. Each problem includes pointer invariants, step-by-step diagrams, common pitfalls, and implementations in C, C++, and JavaScript.