[LeetCode] 1. Two Sum

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

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        if (nums == null || nums.length < 2) {
            return null;
        }
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            int r = target - nums[i];
            if (map.containsKey(r)) {
                return new int[] { map.get(r), i };
            } else {
                map.put(nums[i], i);                
            }
        }
        return null;
    }
}