LeetCode1.java

import java.util.Map;
import java.util.HashMap;

public class LeetCode1 {
    public static void main(String[] args) {
        int[] nums = { 2, 7, 11, 15 };
        int target = 9;
        for (int n : new Solution1_2().twoSum(nums, target)) {
            System.out.println(n);
        }
    }
}

class Solution1_1 {
    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[i] + nums[j] == target) {
                    return new int[] { i, j };
                }
            }
        }
        return new int[0];
    }
}

class Solution1_2 {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> hashtable = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; ++i) {
            if (hashtable.containsKey(target - nums[i])) {
                return new int[] { hashtable.get(target - nums[i]), i };
            }
            hashtable.put(nums[i], i);
        }
        return new int[0];
    }
}