1. 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 the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        int[] ans = new int[2];
        
        for (int i = 0; i<nums.length; i++){
            if(map.containsKey(target-nums[i])) {
                int j = map.get(target-nums[i]);
                ans[0] = j;
                ans[1] = i;
                return ans;
            }else{
                map.put(nums[i], i);
            }
        }
        return null;
    }
}