Overview
Array mein do numbers dhundho jinका sum target ke equal ho — hash map se O(n) solution.
Analogy
Jaise do logon ko dhundho jinki ages ka sum 100 ho — ek khojo, complement check karo.
Step-by-step
- Target - current = complement
- Har element ke liye check karo: complement hash map mein hai?
- Nahi hai toh element map mein daalo
- Mila toh indices return karo
Visual
nums=[2,7,11,15], target=9
2: complement=7 (not in map) -> store {2:0}
7: complement=2 (in map at 0!) -> return [0,1]
Common mistakes
- Same element twice use karna (index same nahi hona chahiye)
- Multiple pairs possible case — kya chahiye pehli ya sabhi?
- Sorted ya unsorted — approach alag hogi
Practice questions
- Two sum hash map se O(n) solve karo
- Two sum sorted array two-pointer se solve karo