Om's Brain

1. Pattern Recognition

3 min read

1️⃣ Step 1: Check Constraints 📌

🔹 Small n (≤ 20)

  • Brute force is valid
  • Backtracking / Recursion
  • Exponential allowed:
    • 2^n, n!
  • Try all possible Permutations & Combinations

🔹 Medium n (10³ → 10⁶)

  • ❌ No brute force
  • Use:
    • Linear Time → O(n) / O(n log n)
  • Patterns
    • Greedy
    • Two pointers
    • Heap
    • Dynamic Programming

🔹 Large n (≥ 10⁷)

  • ❌ No linear solutions
  • Use:
    • O(log n) only
    • Binary search
    • Math formulas
    • O(1) tricks

2️⃣ Step 2: Analyze Input Format

🌳 Tree / BST

  • DFS:
    • All paths
    • Recursive traversal
    • Pre/In/Post order
  • BFS:
    • level-by-level
    • shortest path in unweighted tree
  • Focus:
    • tree properties, parent-child relationships

🌐 Graph (nodes + edges)

  • BFS → shortest path
  • DFS → connected components
  • Union-Find:
    • “connected components”
    • “number of groups”
  • Topological sort:
    • for Dependencies

🧩 2D Grid / Matrix

  • DFS/BFS:
    • Islands problems
  • Union-Find:
    • Connected regions
  • DP:
    • Path problems
  • Consider: 4-directional or 8-directional movement

📊 Sorted Array

🔤 String

  • Two pointers:
    • Palindrome
  • Sliding window:
    • Substrings
  • Trie:
    • Word problems
  • Stack:
    • parentheses/brackets

🔗 Linked List

  • Two pointers → Fast/Slow pointers
  • Dummy node techniques
  • Cycle detection

3️⃣ Step 3: Analyze Output Format

📚 List of Lists (combinations, subsets, paths)

  • Backtracking is almost always the answer
  • Generate all possibilities
  • Use recursion with choice/no-choice pattern

🔢 Single Number Output (max/min profit, cost, ways, jumps):

  • Dynamic Programming for optimization
  • Greedy for local optimal choices
  • Mathematical approach for counting

🔄 Modified Array / String (in-place operations):

  • Two Pointers for in-place modifications

📈 Ordered List (sorted sequence, valid task order):

  • Sorting with custom comparators
  • Topological sort
  • Heap

4️⃣ Step 4: Keyword → Pattern Mapping 🔑

🧠 Dynamic Programming

  • “number of ways”
  • “Maximum/minimum” + “sum/profit/cost”
  • “Can you reach”
  • “Longest/shortest subsequence”
  • “Optimal” or “best”

👉 Two Pointers

  • “palindrome”
  • “sorted array”
  • “target sum”
  • “remove duplicates”

⛰ Heap

  • “k largest/smallest”
  • “top k”
  • “median”
  • “priority”

📚 Stack

  • “Parentheses” or “brackets”
  • “valid expression”
  • “Nested structure”
  • “Undo operations”

📉 Monotonic Stack

  • “Next greater element”
  • “Next smaller element”

🗺 HashMap

  • “Count frequency”
  • “Find duplicates”
  • “Anagram”

🌲 Trie

  • “Word search”
  • “Word prefixes”

⚡ Greedy

  • “Minimum operations”

🔗 Union Find

  • “Connected components”
  • “Number of groups”

🔍 Binary Search

  • “kth element”
  • “Search in sorted”
  • “Minimize maximum”
  • “First/last occurrence”

🔢 Bit Manipulation

  • “XOR” operations
  • ""Single number” problems
  • “power of 2”

📐 Math / Geometry

  • “GCD / LCM”
  • “prime numbers”
  • “Angle calculations”
  • “Coordinate”

🎮 Game Theory

  • “Optimal strategy”
  • “Win/lose scenarios”
  • “Minimax”

🪟 Sliding Window

  • “Substring” with conditions
  • Subarray” with fixed/variable size
  • “Maximum/minimum window”
  • “Contains all”

5️⃣ How to Use This

  • Step-by-step:

    1. Constraints
    2. Input
    3. Output
    4. Keywords

  • Practice strategy:

    • Solve 5–10 problems per pattern
    • Practice only classification
    • Maintain personal keyword notes

Mindmap

Graph View