LeetCode 75 - Complete Solutions Collection

A comprehensive collection of solutions for the LeetCode 75 study plan, covering fundamental algorithms and data structures essential for technical interviews.

🎯 Overview

This repository contains 75 carefully selected LeetCode problems that cover the most important concepts in algorithms and data structures.

📊 Problem Categories

Category	Count	Problems
Array/String	20	Two pointers, sliding window, prefix sum
Two Pointers	8	Array manipulation, collision detection
Sliding Window	7	Fixed/variable window techniques
Stack	5	Monotonic stack, parentheses matching
Binary Tree	12	DFS, BFS, tree traversals
Binary Search Tree	3	Search, insertion, deletion
Graph	5	DFS, BFS, shortest path
Heap/Priority Queue	3	K-largest elements, scheduling
Backtracking	3	Combinations, permutations
Dynamic Programming	12	1D/2D DP, state transitions
Greedy	3	Optimal choices at each step
Bit Manipulation	3	XOR, bit masking
Trie	2	Prefix matching, autocomplete
Intervals	2	Merging, scheduling

🗂️ Solutions Structure

leetcode75/
├── 01-25/           # Easy problems
├── 26-50/           # Medium problems  
├── 51-75/           # Hard problems
└── README.md

🚀 Getting Started

Prerequisites

• Python 3.6+
• No external dependencies required

Running Solutions

# Clone the repository
git clone https://github.com/sachin-kumar-2003/leetcode75.git
cd leetcode75

📋 Problem List

🟢 Easy Problems (25)

#	Problem	Solution	Topics
1	Merge Strings Alternately	01_mergeAlternately.py	Two Pointers
2	Greatest Common Divisor of Strings	02_gcdOfStrings.py	Math
3	Kids With the Greatest Number of Candies	03_kidsWithCandies.py	Array
4	Can Place Flowers	04_canPlaceFlowers.py	Greedy
5	Reverse Words in a String III	05_reverseWords.py	Two Pointers
6	Reverse Vowels of a String	06_reverseVowels.py	Two Pointers
7	Product of Array Except Self	07_productExceptSelf.py	Array
8	Increasing Triplet Subsequence	08_increasingTriplet.py	Greedy
9	String Compression	09_compress.py	Two Pointers
10	Move Zeroes	10_moveZeroes.py	Array
11	Is Subsequence	11_isSubsequence.py	Two Pointers
12	Container With Most Water	12_maxArea.py	Two Pointers
13	Max Number of K-Sum Pairs	13_maxOperations.py	Hash Table
14	Maximum Average Subarray I	14_findMaxAverage.py	Sliding Window
15	Maximum Number of Vowels in a Substring	15_maxVowels.py	Sliding Window
16	Longest Subarray of 1's After Deleting One Element	16_longestOnes.py	Sliding Window
17	Longest Subarray With Maximum Bitwise AND	17_longestSubarray.py	Bit Manipulation
18	Find the Highest Altitude	18_largestAltitude.py	Prefix Sum
19	Find Pivot Index	19_pivotIndex.py	Prefix Sum
20	Find the Difference of Two Arrays	20_findDifference.py	Hash Table
21	Unique Number of Occurrences	21_uniqueOccurrences.py	Hash Table
22	Determine if Two Strings Are Close	22_closeStrings.py	Hash Table
23	Equal Row and Column Pairs	23_equalPairs.py	Hash Table
24	Removing Stars From a String	24_removeStars.py	Stack
25	Asteroid Collision	25_asteroidCollision.py	Stack

🟡 Medium Problems (35)

#	Problem	Solution	Topics
26	Decode String	26_decodeString.py	Stack
27	Recent Counter	27_RecentCounter.py	Queue
28	Predict the Winner	28_predictPartyVictory.py	DP
29	Delete the Middle Node of a Linked List	29_deleteMiddle.py	Linked List
30	Odd Even Linked List	30_oddEvenList.py	Linked List
31	Reverse Linked List	31_reverseList.py	Linked List
32	Maximum Twin Sum of a Linked List	32_pairSum.py	Linked List
33	Maximum Depth of Binary Tree	33_maxDepth.py	Binary Tree
34	Leaf-Similar Trees	34_leafSimilar.py	Binary Tree
35	Count Good Nodes in Binary Tree	35_goodNodes.py	Binary Tree
36	Path Sum III	36_pathSum.py	Binary Tree
37	Longest ZigZag Path in a Binary Tree	37_longestZigZag.py	Binary Tree
38	Lowest Common Ancestor of a Binary Tree	38_lowestCommonAncestor.py	Binary Tree
39	Binary Tree Right Side View	39_rightSideView.py	Binary Tree
40	Maximum Level Sum of a Binary Tree	40_maxLevelSum.py	Binary Tree
41	Search in a Binary Search Tree	41_searchBST.py	Binary Search Tree
42	Delete Node in a BST	42_deleteNode.py	Binary Search Tree
43	Keys and Rooms	43_canVisitAllRooms.py	Graph
44	Number of Provinces	44_findCircleNum.py	Graph
45	Reorder Routes to Make All Paths Lead to the City Zero	45_minReorder.py	Graph
46	Evaluate Division	46_calcEquation.py	Graph
47	Nearest Exit from Entrance in Maze	47_nearestExit.py	Graph
48	Rotting Oranges	48_orangesRotting.py	Graph
49	Kth Largest Element in an Array	49_findKthLargest.py	Heap
50	Smallest Infinite Set	50_SmallestInfiniteSet.py	Heap
51	Maximum Score After Splitting a String	51_maxScore.py	Greedy
52	Total Cost to Hire K Workers	52_totalCost.py	Heap
53	Guess Number Higher or Lower	53_guessNumber.py	Binary Search
54	Successful Pairs of Spells and Potions	54_successfulPairs.py	Binary Search
55	Find Peak Element	55_findPeakElement.py	Binary Search
56	Koko Eating Bananas	56_minEatingSpeed.py	Binary Search
57	Letter Combinations of a Phone Number	57_letterCombinations.py	Backtracking
58	Combination Sum III	58_combinationSum3.py	Backtracking
59	N-th Tribonacci Number	59_tribonacci.py	Dynamic Programming
60	Min Cost Climbing Stairs	60_minCostClimbingStairs.py	Dynamic Programming
61	House Robber	61_rob.py	Dynamic Programming
62	Domino and Tromino Tiling	62_numTilings.py	Dynamic Programming
63	Unique Paths	63_uniquePaths.py	Dynamic Programming
64	Longest Common Subsequence	64_longestCommonSubsequence.py	Dynamic Programming
65	Best Time to Buy and Sell Stock	65_maxProfit.py	Dynamic Programming
66	Edit Distance	66_minDistance.py	Dynamic Programming
67	Counting Bits	67_countBits.py	Bit Manipulation
68	Single Number	68_singleNumber.py	Bit Manipulation
69	Minimum Flips to Make a OR b Equal to c	69_minFlips.py	Bit Manipulation
70	Implement Trie (Prefix Tree)	70_Trie.py	Trie
71	Search Suggestions System	71_suggestedProducts.py	Trie
72	Non-overlapping Intervals	72_eraseOverlapIntervals.py	Intervals
73	Minimum Number of Arrows to Burst Balloons	73_findMinArrowShots.py	Intervals
74	Daily Temperatures	74_dailyTemperatures.py	Stack
75	Online Stock Span	75_StockSpanner.py	Stack

🎯 Problem-Solving Patterns

Array/String Patterns

• Two Pointers: Use two indices moving towards/away from each other
• Sliding Window: Maintain a window of elements with left/right pointers
• Prefix Sum: Pre-calculate cumulative sums for range queries

Tree Patterns

• DFS: Depth-first traversal (preorder, inorder, postorder)
• BFS: Level-order traversal using queue
• Recursion: Natural fit for tree problems

Dynamic Programming Patterns

• 1D DP: Single dimension state
• 2D DP: Two dimensions state (strings, grids)
• State transitions: Build solution from smaller subproblems

🚀 Performance Optimization Tips

Time Optimization

• Avoid nested loops when possible
• Use hash maps for O(1) lookups
• Pre-calculate values that are used multiple times

Space Optimization

• In-place modifications to reduce extra space
• Re-use data structures instead of creating new ones
• Bit manipulation for compact storage

📚 Learning Path

Beginner Path (Problems 1-25)

Start with easy problems to build fundamentals:

1. Array manipulation techniques
2. Two pointer patterns
3. Basic data structures (stack, queue)

Intermediate Path (Problems 26-50)

Progress to medium problems:

1. Linked list operations
2. Tree traversals and operations
3. Graph algorithms (DFS, BFS)

Advanced Path (Problems 51-75)

Master complex algorithms:

1. Dynamic programming patterns
2. Backtracking techniques
3. Advanced data structures (trie, heap)