Overview
Two Sum: array mein do elements dhundo jinका sum target hai, unke indices return karo.
Analogy
Jaise wallet ke coins se exact amount banana—har coin ke liye check karo kitna aur chahiye.
Step-by-step
- Dictionary seen={} banao
- Har number par iterate karo
- complement=target-num calculate karo
- Agar complement seen mein hai toh answer mila, warna num seen mein add karo
Visual
nums=[2,7,11,15], target=9
num=2: need=7, seen={2:0}
num=7: need=2, 2 in {2:0}!
→ [0,1]
Common mistakes
- Sort+two pointer O(n log n) hai aur indices change ho jaate hain
- Same element do baar use nahi kar sakte
Practice questions
- Saare pairs jo target sum dete hain return karo (indices ki jagah values)
- 3Sum tak extend karo