intro practice · coding problem
Two Sum with a hash map
Given an array of integers and a target, return the indices of the two numbers that add to the target. Solve it in a single pass.
The mechanism
A hash map trades memory for time: it turns an O(n) scan for "have I seen this?" into an O(1) lookup. Most array problems that feel quadratic collapse to linear once you ask what you would store in a map keyed by value, complement, or frequency.
Expected evidence
twoSum([2,7,11,15], 9) -> [0,1]
Try this drill in the practice workspace →
Hints when you need them
- For each number, what value would complete the pair?
- Store value -> index as you scan; check for the complement before inserting.
Watch for these mistakes
- Reaching for nested loops before asking "what would a hash map key on?"
- Mutating a collection while iterating it
- Forgetting hash maps have no order — sort separately if order matters