🧠 40-Day Algorithm & LeetCode Learning Review with Exercises

🗓️ Overview

You've just completed 40 days of algorithm mastery, ranging from beginner to advanced LeetCode problems — from arrays, strings, two pointers, and sliding windows to binary search, recursion, dynamic programming (DP), and more!

🧱 Foundation Topics (Days 1–10)

• Big-O Notation: Analyzed time and space complexity.
• Arrays & Strings: Hands-on with iteration, index access, and reverse operations.
• Two Pointers: Efficient techniques for problems like sorted arrays, palindromes.
• Sliding Window: For subarrays and strings (max sum, longest unique substring).

✅ Skills Gained:

• Optimizing loops
• Avoiding brute-force
• Early pattern recognition

📚 Practice Exercises:

1. Two Sum

function twoSum(nums: number[], target: number): number[]
// Input: [2,7,11,15], 9
// Output: [0,1]

2. Best Time to Buy and Sell Stock

function maxProfit(prices: number[]): number
// Input: [7,1,5,3,6,4]
// Output: 5

3. Valid Palindrome

function isPalindrome(s: string): boolean
// Input: "A man, a plan, a canal: Panama"
// Output: true

4. Move Zeroes

function moveZeroes(nums: number[]): void
// Input: [0,1,0,3,12]
// Output: [1,3,12,0,0]

5. Longest Substring Without Repeating Characters

function lengthOfLongestSubstring(s: string): number
// Input: "abcabcbb"
// Output: 3

🔁 Recursion to Binary Search (Days 11–20)

• Recursion: Base cases and call stacks visualized.
• Binary Search: Classical, rotated arrays, and search boundaries.
• Prefix Sums & 2D Grids: Fast sum range queries and matrix manipulations.

✅ Skills Gained:

• Recursion tracing
• Optimizing with pre-processing
• Matrix intuition

📚 Practice Exercises:

1. Binary Search

function search(nums: number[], target: number): number
// Input: [-1,0,3,5,9,12], 9
// Output: 4

2. Search in Rotated Sorted Array

function search(nums: number[], target: number): number
// Input: [4,5,6,7,0,1,2], 0
// Output: 4

3. Koko Eating Bananas (Binary Search on Answer)

function minEatingSpeed(piles: number[], h: number): number
// Input: [3,6,7,11], 8
// Output: 4

4. Sum of Subarray Minimums

function sumSubarrayMins(arr: number[]): number
// Input: [3,1,2,4]
// Output: 17

5. Path Sum (Recursion on Tree)

function hasPathSum(root: TreeNode | null, targetSum: number): boolean
// Input: [5,4,8,11,null,13,4,7,2,null,null,null,1], 22
// Output: true

🔄 Intermediate Patterns (Days 21–30)

• Backtracking: Permutations, combinations, subset generation.
• Dynamic Programming (Easy–Medium):
  • Fibonacci (top-down & bottom-up)
  • House Robber
  • Climbing Stairs variations
• Greedy Thinking: Making the locally optimal choice (Jump Game).

✅ Skills Gained:

• State storage with DP arrays
• Recursive thinking to iterative translation
• Pattern generalization

📚 Practice Exercises:

1. Subsets

function subsets(nums: number[]): number[][]
// Input: [1,2,3]
// Output: [[1,2,3],[1,2],[1,3],[2,3],[1],[2],[3],[]]

2. Permutations

function permute(nums: number[]): number[][]
// Input: [1,2,3]
// Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

3. House Robber

function rob(nums: number[]): number
// Input: [1,2,3,1]
// Output: 4

4. Climbing Stairs

function climbStairs(n: number): number
// Input: 3
// Output: 3

5. Jump Game

function canJump(nums: number[]): boolean
// Input: [2,3,1,1,4]
// Output: true

🧠 Advanced Patterns & DP (Days 31–40)

• Partitioning & State Transitions: Decode Ways, Coin Change, Longest Palindromic Substring.
• Interval DP: Matrix Chain Multiplication, Burst Balloons
• Knapsack Variation: 0/1 and Unbounded Knapsack

✅ Skills Gained:

• Breaking down overlapping subproblems
• Thinking in ranges and partitions
• Space optimization techniques

📚 Practice Exercises:

1. Decode Ways

function numDecodings(s: string): number
// Input: "226"
// Output: 3

2. Coin Change

function coinChange(coins: number[], amount: number): number
// Input: [1,2,5], 11
// Output: 3

3. Palindromic Substrings

function countSubstrings(s: string): number
// Input: "abc"
// Output: 3

4. Burst Balloons

function maxCoins(nums: number[]): number
// Input: [3,1,5,8]
// Output: 167

5. Partition Equal Subset Sum

function canPartition(nums: number[]): boolean
// Input: [1,5,11,5]
// Output: true

📘 Key Learning Patterns

• DP Formulas: dp[i], dp[i][j], and transitions
• Memoization vs Tabulation
• Two Pointer Sliding Strategy
• Binary Search on Answer
• Backtracking Trees

🔍 Tips Going Forward

1. Review your weak spots with a second round.
2. Start daily LeetCode practice: mix difficulty levels.
3. Build a digital note/bookmark of patterns.
4. Begin mock interviews and timed contests.

🧪 Self-Test Challenge

Try solving one problem from each of these categories:

• Sliding Window
• Binary Search (Rotated Array)
• Backtracking (Subsets)
• DP (Knapsack or Palindromes)
• Interval DP (Burst Balloons)

🔮 Up Next

▶️ Day 41: Bit Manipulation — Single Number Problems

You're leveling up — and now it’s time to explore the magic of bits 🧮!