1. Two Sum.java

public class TwoSum {

    /**
     *  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.

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

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

     return [0, 1].

     time : O(n)
     space : O(n)
     * @param nums
     * @param target
     * @return
     */

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

       if (nums == null || nums.length < 2) {
           return new int[]{-1, -1};
       }

       int[] res = new int[]{-1, -1};
       HashMap<Integer, Integer> map = new HashMap<>();
       for (int i = 0; i < nums.length; i++) {
           if (map.containsKey(target - nums[i])) {
               res[0] = map.get(target - nums[i]);
               res[1] = i;
               break;
           }
           map.put(nums[i], i);
       }

       return res;
    }
}