Fundamentals · Easy · Free
Write and explain Two Sum — brute force vs hash map approach.
FresherFundamentalsData Structures & Algorithms
Two Sum
Given array and target, return indices of two numbers that sum to target. Brute force checks all pairs O(n²); hash map stores value→index for O(n) one pass.
java
int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> seen = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
int need = target - nums[i];
if (seen.containsKey(need)) return new int[]{seen.get(need), i};
seen.put(nums[i], i);
}
return new int[]{};
}- Edge: duplicate values, negative numbers
- Space O(n) for map
Say brute force first, then optimize—interviewers track your process.