Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use thesameelement twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0,1].

Solution #1:

Use a hash map to store previously visited numbers and their index, and as we iterate through the number list, find if target - current exist in the hash map.

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