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

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

/*
 * @lc app=leetcode id=1 lang=java
 *
 * [1] Two Sum
 */
class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] res = new int[2];
        if (nums == null || nums.length <= 1) {
            return res;
        }
        Map<Integer, Integer> m = new HashMap<>();

        for (int i = 0; i < nums.length; i++) {
            Integer pst = m.get(nums[i]);
            if (pst == null) {
                m.put(target - nums[i], i);
            } else {
                res[0] = pst;
                res[1] = i;
                break;
            }

        }
        return res;

    }
}