解答1[Java]：

import java.util.HashMap;

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap();

        for (int i = 0; i < nums.length; i++) {
            if (map.get(target - nums[i]) != null) {
                return new int[] { map.get(target - nums[i]), i };
            } else {
                map.put(nums[i], i);
            }
        }

        throw new IllegalArgumentException("No two sum solution");
    }
}