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02-Problems

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README.md

Track 2 — Gates (Problems Bank)

Learn + Solve. Every folder is a Gate. Enter only Gates at or below your Hunter Rank.

Spoiler rule: no finished solutions ship by default. You create solution-1.php.

At a glance
Bank size 238 Gate stubs (climb + Side Quests + Interview Armor)
Order law Gate IDs ascend with difficulty: E → D → C → B → A → S → Side → Armor
Start 01-E-Gates/ — lowest IDs first
Layout Rank-folder / Pattern-category / Gate-NNN-kebab/
Optional 07-Side-Quests/ — not graduation-critical
Armor Linked lists / TreeNode → ../04-Interview-Armor/ (after S on the ID ladder)

How to clear one Gate: open its README.md → Brief → Brute → Tighten → Talk (see root Quick start) → fill solution-1.php → mark ✅ below → update root Status Window.

Folder name: Gate-NNN-kebab-case/ under Rank/Category/ (example: 02-D-Gates/05-Arrays-Hashing/Gate-080-contains-duplicate/).

Category order: within each rank, numbered folders ascend by skill load (easy drills → core patterns). Gate IDs follow that walk.

Difficulty emoji

Every Gate title and map row carries a rank emoji (climb heat, not platform labels):

Emoji Lane Feel
🌱 E-Gates Survive — warmups
📗 D-Gates Core Easy
⚔️ C-Gates Easy → early Medium
🔥 B-Gates Main Medium
💎 A-Gates Stretch
🌑 S-Gates Prestige
🧭 Side Quests Optional detour
🛡️ Interview Armor Lists / trees (late)

Gate ranks

Folder Min Hunter Rank Intent ID range
🌱 01-E-Gates/ E Survive — PHP confidence 001–049
📗 02-D-Gates/ D Core Easy 050–094
⚔️ 03-C-Gates/ C Easy → early Medium 095–132
🔥 04-B-Gates/ B Main Medium set 133–178
💎 05-A-Gates/ A Stretch 179–197
🌑 06-S-Gates/ S Prestige (PHP-fit only) 198–200
🧭 07-Side-Quests/ any Optional — not graduation-critical 201–221
🛡️ ../04-Interview-Armor/ B+ Interview lists/trees 222–238

Gate Source Map (progression order)

Dedup rule: one problem idea → one climb Gate. Dual platform Sources are OK.
Related-but-distinct Gates stay separate and link each other (## Related Gates).
Interview-form twins live in 04-Interview-Armor/ (not a second core Gate).
IDs are sequential by rank — not by import wave.

🌱 01-E-Gates (49)

Gate Title Category Platform XP Status
001 🌱 Sum of Two Numbers 01-Numbers Edabit 25 ☐
002 🌱 Next Number 01-Numbers Edabit 25 ☐
003 🌱 Minutes to Seconds 01-Numbers Edabit 25 ☐
004 🌱 Hours to Seconds 01-Numbers Edabit 25 ☐
005 🌱 Remainder of Two Numbers 01-Numbers Edabit 25 ☐
006 🌱 Divisible by Five 01-Numbers Edabit 25 ☐
007 🌱 Multiple of 100 01-Numbers Edabit 25 ☐
008 🌱 Less Than or Equal to Zero 01-Numbers Edabit 25 ☐
009 🌱 Celsius to Fahrenheit 01-Numbers W3Schools 25 ☐
010 🌱 Area of a Triangle 01-Numbers Edabit 25 ☐
011 🌱 Maximum Triangle Edge 01-Numbers Edabit 25 ☐
012 🌱 Rectangle Perimeter 01-Numbers Edabit 25 ☐
013 🌱 Age to Days 01-Numbers Edabit 25 ☐
014 🌱 The Farm Problem 01-Numbers Edabit 25 ☐
015 🌱 Absolute Difference 01-Numbers w3resource 25 ☐
016 🌱 Sum 1 to N 01-Numbers W3Schools 25 ☐
017 🌱 Factorial 01-Numbers W3Schools 25 ☐
018 🌱 Difference of Squares 01-Numbers Exercism 25 ☐
019 🌱 Even or Odd 02-Control-Flow W3Schools 25 ☐
020 🌱 FizzBuzz 02-Control-Flow W3Schools 25 ☐
021 🌱 Keep Up the Hoop 02-Control-Flow Codewars 25 ☐
022 🌱 Multiplication Table for Number 02-Control-Flow Codewars 25 ☐
023 🌱 Grade Decision 02-Control-Flow HackerRank 25 ☐
024 🌱 Sum With a Loop 02-Control-Flow HackerRank 25 ☐
025 🌱 Factorial Table Lines 02-Control-Flow w3resource 25 ☐
026 🌱 Reverse String 03-Strings LeetCode 25 ☐
027 🌱 Count Vowels 03-Strings Original 25 ☐
028 🌱 Word Counter 03-Strings W3Schools 25 ☐
029 🌱 First and Last Character 03-Strings W3Schools 25 ☐
030 🌱 Return Something to Me 03-Strings Edabit 25 ☐
031 🌱 Is the String Empty 03-Strings Edabit 25 ☐
032 🌱 Concatenate Name 03-Strings Edabit 25 ☐
033 🌱 Convert a String to an Array 03-Strings Codewars 25 ☐
034 🌱 Remove String Spaces 03-Strings Codewars 25 ☐
035 🌱 String Concat Parts 03-Strings HackerRank 25 ☐
036 🌱 Length and Uppercase 03-Strings HackerRank 25 ☐
037 🌱 String Stats 03-Strings w3resource 25 ☐
038 🌱 Two Fer 03-Strings Exercism 25 ☐
039 🌱 Sum Array Elements 04-PHP-Arrays-Basics Original 25 ☐
040 🌱 Find Max and Min 04-PHP-Arrays-Basics Original 25 ☐
041 🌱 Average Score 04-PHP-Arrays-Basics W3Schools 25 ☐
042 🌱 Count Matches 04-PHP-Arrays-Basics W3Schools 25 ☐
043 🌱 First Array Element 04-PHP-Arrays-Basics Edabit 25 ☐
044 🌱 Reverse an Array 04-PHP-Arrays-Basics Edabit 25 ☐
045 🌱 Associative Value Sum 04-PHP-Arrays-Basics HackerRank 25 ☐
046 🌱 2D Matrix Sum 04-PHP-Arrays-Basics HackerRank 25 ☐
047 🌱 Array Values Product 04-PHP-Arrays-Basics w3resource 25 ☐
048 🌱 Unique Then Sort 04-PHP-Arrays-Basics w3resource 25 ☐
049 🌱 Resistor Color 04-PHP-Arrays-Basics Exercism 25 ☐

📗 02-D-Gates (45)

Gate Title Category Platform XP Status
050 📗 Leap Year 01-Numbers W3Schools 40 ☐
051 📗 Prime Check 01-Numbers W3Schools 40 ☐
052 📗 Breaking Chocolate Problem 01-Numbers Codewars 40 ☐
053 📗 Descending Order 01-Numbers Codewars 40 ☐
054 📗 Square Every Digit 01-Numbers Codewars 40 ☐
055 📗 Palindrome Number 01-Numbers LeetCode 40 ☐
056 📗 Reverse Integer 01-Numbers LeetCode 40 ☐
057 📗 Plus One 01-Numbers LeetCode 40 ☐
058 📗 JSON Encode User Map 02-JSON w3resource 40 ☐
059 📗 JSON Decode Sum Scores 02-JSON w3resource 40 ☐
060 📗 Format Date Y-m-d 03-Date w3resource 40 ☐
061 📗 Gigasecond 03-Date Exercism 40 ☐
062 📗 Valid Palindrome 04-Strings LeetCode 40 ☐
063 📗 Reverse Words 04-Strings W3Schools 40 ☐
064 📗 Most Common Character 04-Strings W3Schools 40 ☐
065 📗 Highest and Lowest 04-Strings Codewars 40 ☐
066 📗 Disemvowel Trolls 04-Strings Codewars 40 ☐
067 📗 Digits Only Check 04-Strings w3resource 40 ☐
068 📗 Raindrops 04-Strings Exercism 40 ☐
069 📗 Bob 04-Strings Exercism 40 ☐
070 📗 Hamming Distance 04-Strings Exercism 40 ☐
071 📗 Isogram 04-Strings Exercism 40 ☐
072 📗 Pangram 04-Strings Exercism 40 ☐
073 📗 RNA Transcription 04-Strings Exercism 40 ☐
074 📗 Acronym 04-Strings Exercism 40 ☐
075 📗 Isomorphic Strings 04-Strings LeetCode 40 ☐
076 📗 Ransom Note 04-Strings LeetCode 40 ☐
077 📗 Is Subsequence 04-Strings LeetCode 40 ☐
078 📗 Two Sum 05-Arrays-Hashing LeetCode 40 ☐
079 📗 Valid Anagram 05-Arrays-Hashing LeetCode 40 ☐
080 📗 Contains Duplicate 05-Arrays-Hashing LeetCode 40 ☐
081 📗 Move Zeros to End 05-Arrays-Hashing W3Schools 40 ☐
082 📗 Letter Frequency 05-Arrays-Hashing W3Schools 40 ☐
083 📗 Second Largest 05-Arrays-Hashing W3Schools 40 ☐
084 📗 Running Sum 05-Arrays-Hashing W3Schools 40 ☐
085 📗 Two Oldest Ages 05-Arrays-Hashing Codewars 40 ☐
086 📗 Small Enough 05-Arrays-Hashing Codewars 40 ☐
087 📗 Lost Number in Sequence 05-Arrays-Hashing Codewars 40 ☐
088 📗 Scrabble Score 05-Arrays-Hashing Exercism 40 ☐
089 📗 High Scores 05-Arrays-Hashing Exercism 40 ☐
090 📗 Majority Element 05-Arrays-Hashing LeetCode 40 ☐
091 📗 Single Number 05-Arrays-Hashing LeetCode 40 ☐
092 📗 Happy Number 05-Arrays-Hashing LeetCode 40 ☐
093 📗 Contains Duplicate II 05-Arrays-Hashing LeetCode 40 ☐
094 📗 Find Pivot Index 05-Arrays-Hashing LeetCode 40 ☐

⚔️ 03-C-Gates (38)

Gate Title Category Platform XP Status
095 ⚔️ Word Frequency 01-Strings W3Schools 60 ☐
096 ⚔️ Stop Spinning My Words 01-Strings Codewars 60 ☐
097 ⚔️ Create Phone Number 01-Strings Codewars 60 ☐
098 ⚔️ Who Likes It 01-Strings Codewars 60 ☐
099 ⚔️ Zigzag Conversion 01-Strings LeetCode 60 ☐
100 ⚔️ Remove Duplicates Keep Order 02-Arrays W3Schools 60 ☐
101 ⚔️ Rotate Left 02-Arrays W3Schools 60 ☐
102 ⚔️ Merge Sorted Lists 02-Arrays W3Schools 60 ☐
103 ⚔️ Longest Streak 02-Arrays W3Schools 60 ☐
104 ⚔️ Maximum Subarray Sum 02-Arrays W3Schools 60 ☐
105 ⚔️ Equal Sides Of An Array 02-Arrays Codewars 60 ☐
106 ⚔️ Array Diff 02-Arrays Codewars 60 ☐
107 ⚔️ Bit Counting 02-Arrays Codewars 60 ☐
108 ⚔️ Pair Closest to Zero Sum 02-Arrays w3resource 60 ☐
109 ⚔️ Product of Array Except Self 02-Arrays LeetCode 60 ☐
110 ⚔️ Rotate Array 02-Arrays LeetCode 60 ☐
111 ⚔️ Partition Labels 02-Arrays LeetCode 60 ☐
112 ⚔️ Counting Bits 02-Arrays LeetCode 60 ☐
113 ⚔️ Merge Sorted Array 02-Arrays LeetCode 60 ☐
114 ⚔️ Flatten Array 03-Nested-Arrays Exercism 60 ☐
115 ⚔️ Valid Parentheses 04-Stack LeetCode 60 ☐
116 ⚔️ Daily Temperatures 04-Stack LeetCode 60 ☐
117 ⚔️ Decode String 04-Stack LeetCode 60 ☐
118 ⚔️ Group Anagrams 05-Hashing LeetCode 60 ☐
119 ⚔️ Longest Consecutive Sequence 05-Hashing LeetCode 60 ☐
120 ⚔️ Subarray Sum Equals K 05-Hashing LeetCode 60 ☐
121 ⚔️ Container With Most Water 06-Two-Pointers LeetCode 60 ☐
122 ⚔️ Longest Common Prefix 06-Two-Pointers W3Schools 60 ☐
123 ⚔️ 3Sum 06-Two-Pointers LeetCode 60 ☐
124 ⚔️ Sort Colors 06-Two-Pointers LeetCode 60 ☐
125 ⚔️ Squares of a Sorted Array 06-Two-Pointers LeetCode 60 ☐
126 ⚔️ Two Sum II - Input Array Is Sorted 06-Two-Pointers LeetCode 60 ☐
127 ⚔️ Best Time to Buy and Sell Stock 07-Sliding-Window LeetCode 60 ☐
128 ⚔️ Longest Substring Without Repeating Characters 07-Sliding-Window LeetCode 60 ☐
129 ⚔️ Find All Anagrams in a String 07-Sliding-Window LeetCode 60 ☐
130 ⚔️ Minimum Size Subarray Sum 07-Sliding-Window LeetCode 60 ☐
131 ⚔️ Longest Repeating Character Replacement 07-Sliding-Window LeetCode 60 ☐
132 ⚔️ Maximum Average Subarray I 07-Sliding-Window LeetCode 60 ☐

🔥 04-B-Gates (46)

Gate Title Category Platform XP Status
133 🔥 Binary Search 01-Binary-Search LeetCode 90 ☐
134 🔥 Search Insert Position 01-Binary-Search LeetCode 90 ☐
135 🔥 Search a 2D Matrix 01-Binary-Search LeetCode 90 ☐
136 🔥 Search in Rotated Sorted Array 01-Binary-Search LeetCode 90 ☐
137 🔥 Find Minimum in Rotated Sorted Array 01-Binary-Search LeetCode 90 ☐
138 🔥 Find First and Last Position of Element in Sorted Array 01-Binary-Search LeetCode 90 ☐
139 🔥 Find Peak Element 01-Binary-Search LeetCode 90 ☐
140 🔥 Search a 2D Matrix II 01-Binary-Search LeetCode 90 ☐
141 🔥 Koko Eating Bananas 01-Binary-Search LeetCode 90 ☐
142 🔥 Next Permutation 02-Arrays-Medium LeetCode 90 ☐
143 🔥 Find the Duplicate Number 03-Hashing-Medium LeetCode 90 ☐
144 🔥 Valid Sudoku 03-Hashing-Medium LeetCode 90 ☐
145 🔥 Top K Frequent Elements 04-Heap-TopK LeetCode 90 ☐
146 🔥 Kth Largest Element in an Array 04-Heap-TopK LeetCode 90 ☐
147 🔥 Merge Intervals 05-Intervals LeetCode 90 ☐
148 🔥 Insert Interval 05-Intervals LeetCode 90 ☐
149 🔥 Non-overlapping Intervals 05-Intervals LeetCode 90 ☐
150 🔥 Min Stack 06-Stack-Design LeetCode 90 ☐
151 🔥 Simplify Path 06-Stack-Design LeetCode 90 ☐
152 🔥 Evaluate Reverse Polish Notation 06-Stack-Design LeetCode 90 ☐
153 🔥 LRU Cache 07-Cache-Design LeetCode 90 ☐
154 🔥 Insert Delete GetRandom O(1) 07-Cache-Design LeetCode 90 ☐
155 🔥 Maximum Depth (Nested Structure) 08-Nested-Trees LeetCode 90 ☐
156 🔥 Implement Trie (Prefix Tree) 09-Trie-Design LeetCode 90 ☐
157 🔥 Jump Game 10-Greedy LeetCode 90 ☐
158 🔥 Jump Game II 10-Greedy LeetCode 90 ☐
159 🔥 Rotate Image 11-Matrix LeetCode 90 ☐
160 🔥 Set Matrix Zeroes 11-Matrix LeetCode 90 ☐
161 🔥 Spiral Matrix 11-Matrix LeetCode 90 ☐
162 🔥 Subsets 12-Backtracking LeetCode 90 ☐
163 🔥 Permutations 12-Backtracking LeetCode 90 ☐
164 🔥 Combination Sum 12-Backtracking LeetCode 90 ☐
165 🔥 Generate Parentheses 12-Backtracking LeetCode 90 ☐
166 🔥 Letter Combinations of a Phone Number 12-Backtracking LeetCode 90 ☐
167 🔥 Meeting Rooms II 05-Intervals LeetCode 90 ☐
168 🔥 Time Based Key-Value Store 01-Binary-Search LeetCode 90 ☐
169 🔥 Gas Station 10-Greedy LeetCode 90 ☐
170 🔥 Asteroid Collision 06-Stack-Design LeetCode 90 ☐
171 🔥 Moving Average from Data Stream 13-Backend-Design LeetCode 90 ☐
172 🔥 Encode and Decode TinyURL 13-Backend-Design LeetCode 90 ☐
173 🔥 Design Authentication Manager 13-Backend-Design LeetCode 90 ☐
174 🔥 First Bad Version 01-Binary-Search LeetCode 90 ☐
175 🔥 Capacity To Ship Packages Within D Days 01-Binary-Search LeetCode 90 ☐
176 🔥 Task Scheduler 04-Heap-TopK LeetCode 90 ☐
177 🔥 Car Pooling 05-Intervals LeetCode 90 ☐
178 🔥 My Calendar I 05-Intervals LeetCode 90 ☐

💎 05-A-Gates (19)

Gate Title Category Platform XP Status
179 💎 Number of Islands 01-Graphs LeetCode 130 ☐
180 💎 Course Schedule 01-Graphs LeetCode 130 ☐
181 💎 Course Schedule II 01-Graphs LeetCode 130 ☐
182 💎 Number of Provinces 01-Graphs LeetCode 130 ☐
183 💎 Rotting Oranges 01-Graphs LeetCode 130 ☐
184 💎 Climbing Stairs 02-DP-Intro LeetCode 130 ☐
185 💎 House Robber 02-DP-Intro LeetCode 130 ☐
186 💎 House Robber II 02-DP-Intro LeetCode 130 ☐
187 💎 Coin Change 02-DP-Intro LeetCode 130 ☐
188 💎 Unique Paths 02-DP-Intro LeetCode 130 ☐
189 💎 Minimum Path Sum 02-DP-Intro LeetCode 130 ☐
190 💎 Maximum Product Subarray 02-DP-Intro LeetCode 130 ☐
191 💎 Word Break 02-DP-Intro LeetCode 130 ☐
192 💎 Longest Increasing Subsequence 02-DP-Intro LeetCode 130 ☐
193 💎 Longest Common Subsequence 02-DP-Intro LeetCode 130 ☐
194 💎 Partition Equal Subset Sum 02-DP-Intro LeetCode 130 ☐
195 💎 Coin Change II 02-DP-Intro LeetCode 130 ☐
196 💎 Trapping Rain Water 02-DP-Intro LeetCode 130 ☐
197 💎 Word Search 03-Backtracking-Harder LeetCode 130 ☐

🌑 06-S-Gates (3)

Gate Title Category Platform XP Status
198 🌑 Minimum Window Substring 01-Hard-PHP-Fit LeetCode 200 ☐
199 🌑 Edit Distance 01-Hard-PHP-Fit LeetCode 200 ☐
200 🌑 Sliding Window Maximum 01-Hard-PHP-Fit LeetCode 200 ☐

🧭 07-Side-Quests (21)

Gate Title Folder Platform XP Status
201 🧭 First and Last Digit 01-CodeChef CodeChef 25 ☐
202 🧭 Sum of Digits 01-CodeChef CodeChef 25 ☐
203 🧭 Reverse The Number 01-CodeChef CodeChef 25 ☐
204 🧭 Lucky Four 01-CodeChef CodeChef 25 ☐
205 🧭 Chef and Operators 01-CodeChef CodeChef 25 ☐
206 🧭 Second Largest of Three 01-CodeChef CodeChef 40 ☐
207 🧭 ATM Withdrawal 01-CodeChef CodeChef 40 ☐
208 🧭 Watermelon 02-Codeforces Codeforces 25 ☐
209 🧭 Way Too Long Words 02-Codeforces Codeforces 25 ☐
210 🧭 Team 02-Codeforces Codeforces 25 ☐
211 🧭 Bit++ 02-Codeforces Codeforces 25 ☐
212 🧭 Domino piling 02-Codeforces Codeforces 25 ☐
213 🧭 Next Round 02-Codeforces Codeforces 25 ☐
214 🧭 Beautiful Matrix 02-Codeforces Codeforces 25 ☐
215 🧭 Helpful Maths 02-Codeforces Codeforces 25 ☐
216 🧭 Stones on the Table 02-Codeforces Codeforces 25 ☐
217 🧭 Bear and Big Brother 02-Codeforces Codeforces 25 ☐
218 🧭 Wrong Subtraction 02-Codeforces Codeforces 25 ☐
219 🧭 Kefa and First Steps 02-Codeforces Codeforces 40 ☐
220 🧭 Selection Sort Steps 03-W3Resource-Extras w3resource 40 ☐
221 🧭 Bubble Sort Swap Count 04-W3Schools-Extras W3Schools 40 ☐

🛡️ Interview Armor (17)

Sealed until B-Rank+ on the PHP-fit climb. IDs follow Side Quests on the global ladder.

Gate Title Part Platform XP Status
222 🛡️ Reverse Linked List Part-1-Linked-Lists-Interview LeetCode 90 ☐
223 🛡️ Merge Two Sorted Lists Part-1-Linked-Lists-Interview LeetCode 90 ☐
224 🛡️ Linked List Cycle Part-1-Linked-Lists-Interview LeetCode 90 ☐
225 🛡️ Remove Nth Node From End of List Part-1-Linked-Lists-Interview LeetCode 90 ☐
226 🛡️ Linked List Cycle II Part-1-Linked-Lists-Interview LeetCode 90 ☐
227 🛡️ Palindrome Linked List Part-1-Linked-Lists-Interview LeetCode 90 ☐
228 🛡️ Add Two Numbers Part-1-Linked-Lists-Interview LeetCode 90 ☐
229 🛡️ Swap Nodes in Pairs Part-1-Linked-Lists-Interview LeetCode 90 ☐
230 🛡️ Invert Binary Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
231 🛡️ Symmetric Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
232 🛡️ Validate Binary Search Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
233 🛡️ Binary Tree Level Order Traversal Part-2-TreeNode-Interview-Form LeetCode 90 ☐
234 🛡️ Diameter of Binary Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
235 🛡️ Kth Smallest Element in a BST Part-2-TreeNode-Interview-Form LeetCode 90 ☐
236 🛡️ Lowest Common Ancestor of a Binary Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
237 🛡️ Maximum Depth of Binary Tree Part-2-TreeNode-Interview-Form LeetCode 90 ☐
238 🛡️ Binary Tree Inorder Traversal Part-2-TreeNode-Interview-Form LeetCode 90 ☐

Session cycle

  1. Confirm Rank unlock
  2. Read System Brief + Learn First
  3. Attempt alone
  4. Coach only if stuck (repo-ideas/21-…)
  5. CHECK → mark ✅ → XP → update Status Window

Before Instant Dungeons (Track 3)

Usually needs B-Rank+ and core E/D/C/B clears in order.