Two Sum

Optimized for runtime.
Time O(n)
Memory O(n)

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] ret = new int[2];
        HashMap < Integer, Integer > map = new HashMap < Integer, Integer > ();
        for (int i = 0; i < nums.length; i++) {
            if (map.get(nums[i]) != null) {
                ret[0] = map.get(nums[i]);
                ret[1] = i;
                return ret;
            }
            map.put(target - nums[i], i);
        }
        return ret;
    }
}