TwoSum.java

package LeetCodeJava.HashTable;

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

public class TwoSum {

    // V1
    public int[] twoSum_1(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            map.put(nums[i], i);
        }
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement) && map.get(complement) != i) {
                return new int[] { i, map.get(complement) };
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }

    // V2
    public int[] twoSum_2(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[] { map.get(complement), i };
            }
            map.put(nums[i], i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }

    public int[] twoSum_3(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++){
            int diff = target - nums[i];
            if (map.containsKey(diff)){
                return new int[]{map.get(diff), i};
            }
            map.put(nums[i], i);
        }
        return null;
    }
}