TwoSum.java

import java.util.HashMap;

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// Given an array of integers, return indices of the two numbers such that they add up to a specific target.

// You may assume that each input would have exactly one solution, and you may not use the same element twice.

// Example:

// Given nums = [2, 7, 11, 15], target = 9,

// Because nums[0] + nums[1] = 2 + 7 = 9,
// return [0, 1].


class Solution {
  public int[] twoSum(int[] nums, int target) {
    int[] result = new int[2];
    if (nums == null || nums.length == 1) { return result; }
    HashMap<Integer, Integer> lookup = new HashMap<>();

    for (int i = 0; i < nums.length; i++) {
      int number = nums[i];
      
      if (lookup.get(number) != null && lookup.get(number) != i) {
        result[0] = lookup.get(number);
        result[1] = i;
        return result;
      } else {
        lookup.put(target - number, i);
      }
    }
    
    return result;
  }
}