Difficulty Progression

A structured, ordered approach to tackling all topics — from zero to interview-ready. Follow the phases in order. Don’t skip ahead.

Phase 0 — Foundations (Week 1–2)

Master these first. Every other topic builds on them.

Order Topic Key Concept First Problem
1 Complexity Big O, time/space analysis Understand before coding anything
2 HashMap O(1) lookup, frequency counting #1 Two Sum
3 Two Pointers Left/right converging #125 Valid Palindrome
4 Sliding Window Moving subarray #643 Max Avg Subarray I
5 Stack LIFO, next greater element #20 Valid Parentheses

Goal: Solve 2–3 Easy problems per topic. No Hard problems yet.

Phase 1 — Core Data Structures (Week 3–4)

Order Topic Key Concept First Problem
6 Recursion Base case + subproblem #509 Fibonacci Number
7 Linked List Two pointers on list, dummy node #206 Reverse Linked List
8 Tree DFS Pre/in/post order #104 Max Depth of Binary Tree
9 Tree BFS Level order, queue #102 Binary Tree Level Order
10 BST Search, insert, validate #700 Search in a BST
11 Heap Top K elements, priority queue #703 Kth Largest in Stream

Goal: Mix of Easy and Medium. Start attempting Mediums independently.

Phase 2 — Algorithms (Week 5–7)

Order Topic Key Concept First Problem
12 Binary Search Eliminate half, boundary conditions #704 Binary Search
13 Graph BFS Grid traversal, connected components #200 Number of Islands
14 Graph DFS Cycle detection, DFS on explicit graph #133 Clone Graph
15 Greedy Sort by key, exchange argument #455 Assign Cookies
16 Backtracking Make/recurse/undo template #78 Subsets
17 Divide & Conquer Merge sort, quick select #215 Kth Largest Element

Goal: All Mediums. Begin attempting Easy Hards.

Phase 3 — Dynamic Programming (Week 8–10)

DP is the hardest topic. Spend 2–3 weeks here.

Order Sub-topic First Problem Hard version
1D DP Single variable #70 Climbing Stairs #139 Word Break
1D DP Skip/take decision #198 House Robber #213 House Robber II
1D DP Unbounded knapsack #322 Coin Change #518 Coin Change II
2D DP Subsequences #300 Longest Inc. Subsequence #1143 LCS
2D DP Grid DP #62 Unique Paths #64 Min Path Sum

Goal: 15–20 DP problems minimum. Understand the pattern for each sub-type.

Phase 4 — Advanced Graphs (Week 11–12)

Order Topic Key Concept First Problem
18 Topological Sort DAG ordering, cycle detection #207 Course Schedule
19 Union Find DSU template, path compression #547 Number of Provinces
20 Dijkstra Weighted shortest path #743 Network Delay Time
21 Bellman-Ford Negative weights, k-stops #787 Cheapest Flights K Stops
22 A* Search Heuristic-guided search #1091 Shortest Path Binary Matrix

Phase 5 — Advanced Data Structures (Week 13–14)

Order Topic Use Case First Problem
23 Trie Prefix matching #208 Implement Trie
24 Segment Tree Range queries + updates #307 Range Sum Query Mutable
25 Fenwick Tree Prefix sums + updates #307 Range Sum Query Mutable
26 AVL Trees Self-balancing BST Concept + #1382
27 Suffix Array Advanced string problems #1044 Longest Dup Substring
Week Focus Problems/Week
1–2 Phase 0 (foundations) 10–15 Easy
3–4 Phase 1 (data structures) 15–20 Easy+Medium
5–7 Phase 2 (algorithms) 20–25 Medium
8–10 Phase 3 (DP) 20–25 Medium+Hard
11–12 Phase 4 (advanced graphs) 15–20 Medium+Hard
13–14 Phase 5 (advanced DS) 10–15 Hard

Progress Checkpoints

Before moving to the next phase, make sure you can:

  • After Phase 0: Solve any Easy problem involving arrays, strings, or hash maps within 15 minutes
  • After Phase 1: Implement a BST, traverse a tree in all orders, and reverse a linked list from memory
  • After Phase 2: Solve most LeetCode Mediums in graph, binary search, and backtracking topics
  • After Phase 3: Recognize DP problems, define the state, and write the recurrence before coding
  • After Phase 4: Implement Dijkstra and topological sort from memory
  • After Phase 5: Know when each advanced DS applies and implement the core template

Problem Count Target for Interviews

Level Problems Solved Ready For
50 Mostly Easy Initial screening, startups
100 Easy + Medium Most mid-level roles
150 Mix + some Hard FAANG / top-tier roles
200+ All patterns + Hard Senior / Staff interviews

Quality > quantity. 75 well-understood problems beat 200 problems you half-remember.