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].

  

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