1 两数之和

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