2 Sum

Given an array of unsorted integers and a target integer x, write a method int[] twoSum(int[] a, int x) that returns the indices of two numbers that sum to x.

Example: given x = 10 and int[] a = [9,2,1,4,7], your function should return [0,2] in any order. If no such combination exists that sum to x, your method return [-1,-1].

Solution

public int[] twoSum(int[] nums, int target) {

    // store complements in a hashmap. key is complement, value is index
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();

    for(int i = 0; i < nums.length; i++) {
        if(map.containsKey(nums[i])) {
            return new int[]{map.get(nums[i]), i};
        } else {
            map.put(target-nums[i], i);
        }
    }

    return new int[]{-1,-1};
}