hashmap<num, index>

class Solution {
    /**
     * hashmap<num, index>
     */
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target - nums[i])) {
                return new int[]{i, map.get(target - nums[i])};
            } else {
                map.put(nums[i], i);
            }
        }
        // 不可能到达的位置
        return null;
    }
}