Two Sum - https://leetcode.com/problems/two-sum/description/

gistfile1.txt

class Solution {
    public int[] twoSum(int[] nums, int target) {
      // Tests:
      // 1. Empty array
      // 2. 1 element
      // 3. Negative vs positive
      // Exactly One solution
      
      if (nums == null || nums.length == 0) {
        return null;
      }
      
      Map<Integer, Integer> numToIndex = new HashMap<Integer, Integer>();
      
      final int len = nums.length;
      for (int i = 0; i < len; i++) {
        int num = nums[i];
        int complement = target - num;
        if (numToIndex.containsKey(complement)) {
          return new int[] {numToIndex.get(complement), i};
        }
        
        numToIndex.put(num, i);
      }
      
      return null;
    }
}