LeetCode 121 – Best Time to Buy and Sell Stock
Max profit from one buy/sell.
Single pass; track minPriceSoFar and best = max(best, price - minPriceSoFar).
Sliding “anchor” is the min so far.
Time O(n), Space O(1).
Tags: One-pass, Sliding Anchor.
LeetCode 3 – Longest Substring w/o Repeats
Length of longest substring with all unique chars.
Expand right, count chars; when a char repeats, shrink left until valid.
Map/array to store last index or freq.
Maintain best = max(best, windowLen).
Time O(n), Space O(Σ).
Tags: Sliding Window, Hash Map.
LeetCode 424 – Longest Repeating Char Replacement
With at most k replacements, longest substring of same char.
Window expands; track maxFreq of any char in window.
If windowLen - maxFreq > k, shrink from left.
Answer is max window size seen.
Time O(n), Space O(Σ).
Tags: Sliding Window, Frequency, “keep maxFreq”.
LeetCode 567 – Permutation in String
Does s2 contain a permutation of s1?
Need a window of size len(s1) with identical char counts.
Maintain freq diff (or two arrays) and a matches counter.
Slide fixed-size window over s2; if all counts zero / matches==Σ → true.
Time O(n + m), Space O(Σ).
Tags: Fixed-size Window, Frequency Match.
LeetCode 76 – Minimum Window Substring
Smallest window in s containing all chars of t.
Need counts of t. Expand right, decrement need; when all met, try to shrink from left to minimal.
Track best [L,R].
Careful with duplicates and zero/negative counts.
Time O(n), Space O(Σ).
Tags: Sliding Window, Two Pointers, “expand then contract”.
LeetCode 239 – Sliding Window Maximum
Max of each window of size k.
Monotonic deque of indices (values decreasing).
Pop back while nums[i] ≥ tail; push i.
Pop front if out of window (i - k + 1).
Front is max for current window.
Time O(n), Space O(k).
Tags: Monotonic Queue, Fixed-size Window.
