LeetCode Hard.
Solved Systematically.

Stop guessing. Learn the 20 patterns that cover 80% of all Hard problems — then practice 100 walkthroughs until pattern recognition becomes instinct.

📅 Weeks 13–16 🎯 20 Core Patterns 💀 100 Hard Walkthroughs ⏱ Mock Contests
Phase 5 Modules
Four-Part System
Pattern recognition → Walkthrough practice → Timed simulation → Volume practice
20

Core Pattern Cheatsheet

The 20 patterns that appear in 80% of Hard problems — now with a decision flowchart to route any problem to its pattern, plus an 8-question recognition speed drill.

20 patterns · flowchart · speed drill Open →
100

Hard Problem Walkthroughs

Think-aloud walkthroughs of classic Hards — including an interactive Trapping Rain Water visualizer and a two-heaps median-from-stream deep dive.

Walkthroughs + live visualizer Open →
⏱

Mock Contest Simulator

Generate randomized timed contests from the 615-problem bank — difficulty presets, countdown clock, results history — plus curated sets and the full strategy guide.

contest generator · 3 curated sets · strategy guide Open →
615

The Problem Bank

615 hand-picked LeetCode problems across 16 topics, sorted Easy → Medium → Hard — each with a one-line pattern cue, a reference C++ solution, and a direct link to solve it.

16 topics · 615 C++ solutions Open →
Pattern Cheatsheet Preview
20 Patterns to Rule Them All
Click any pattern to see the full template and practice problems
01 ·
Sliding Window
Subarray/substring problems
3M5H
02 ·
Two Pointers
Sorted arrays, pairs, triplets
3M4H
03 ·
Fast & Slow Pointers
Cycle detection in linked lists
2M3H
04 ·
Merge Intervals
Overlapping ranges
4M5H
05 ·
Cyclic Sort
Range [1,n] arrays
2M3H
06 ·
LL In-place Reversal
Sublist reversal problems
2M5H
07 ·
Tree BFS
Level-order traversal
4M5H
08 ·
Tree DFS
Path sum, tree structure
4M5H
09 ·
Two Heaps
Median tracking, splits
2M5H
10 ·
Subsets
All permutations, combos
3M4H
11 ·
Modified Binary Search
Rotated/infinite arrays
3M5H
12 ·
Bitwise XOR
Missing/duplicate numbers
2M3H
13 ·
Top K Elements
Heap + sorting hybrid
3M4H
14 ·
K-way Merge
Merge sorted lists
2M5H
15 ·
Knapsack DP
0/1 choice problems
3M5H
16 ·
Unbounded Knapsack
Coin change variants
3M4H
17 ·
Fibonacci DP
House robber, staircase
2M4H
18 ·
Palindromic DP
LPS, palindrome count
2M5H
19 ·
LCS Pattern
Edit distance, matching
3M5H
20 ·
Topological Sort
Task scheduling, DAG
3M5H
Hard Problem Preview
Sample Walkthroughs
Each problem: intuition → pattern → dry run → clean code
Featured Hard Problems
100 total in full module →
#84
Largest Rectangle in Histogram HARD
Pattern: Monotonic Stack
Given an array of bar heights, find the largest rectangle. Key insight: for each bar, find the nearest shorter bar on both sides using a monotonic stack.
StackArrayO(n)
#76
Minimum Window Substring HARD
Pattern: Sliding Window
Find smallest window in s containing all chars of t. Use two pointers + frequency map. Shrink from left when all chars covered, expand right otherwise.
Sliding WindowHashMapO(n)
#295
Find Median from Data Stream HARD
Pattern: Two Heaps
Maintain two heaps: max-heap for lower half, min-heap for upper half. Balance their sizes to get median in O(1). Insert in O(log n).
Two HeapsO(log n)Design
#124
Binary Tree Maximum Path Sum HARD
Pattern: Tree DFS
At each node, a path can: go up to parent, bend through the node (left + node + right), or just take one branch. Track global max, return best branch to parent.
Tree DFSRecursionO(n)

Mock Contest Simulator

8
Mock Contests
90
Minutes Each
3
Problems/Contest

Simulate real LeetCode contest pressure. Timer counts down. After each contest, see which patterns you missed and get targeted practice.

Start Mock Contest →
← Phase 4: Advanced 🏠 Dashboard