Two Pointers Algorithm – Intuition & Patterns

🧠 What is the Two Pointers Technique?

The two pointers algorithm uses two indices that move towards each other (or in the same direction) to scan through data — typically an array or string — to find solutions faster than nested loops.

I like to call this the squeeze algorithm — because the pointers “squeeze” the problem space from both ends until the condition is met.

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🔍 Where is it Used?

• Sorted arrays (common in questions asking for pair sums)
• Strings or arrays with substructure (e.g., palindromes, reverse operations)
• Finding or optimizing subarrays or subsequences
• Eliminating nested loops when scanning with start-end pairs

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🧩 Types of Movement

1. Same Direction

   • Start both pointers at the beginning
   • Example: slow/fast runners, merging sorted arrays

2. Opposite Direction

   • One pointer at start, the other at end
   • Ideal for sum-based or palindrome problems

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🪄 Why it Works

• Reduces time complexity from O(n²) to O(n) in many problems
• Works well with sorted data or problems where left/right symmetry helps
• Simple to implement but powerful when identifying the correct window or combination

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💡 Key Insight

If a brute-force solution requires nested loops comparing two values at a time — think if those comparisons can be done by moving two pointers instead.

✅ Efficient
✅ Clean logic
✅ Lower space usage