1.两数之和

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

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