1. Two Sum

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

  public int[] twoSumTwoPointer(int[] nums, int target) {
        N[] ns = new N[nums.length];
        for(int i = 0; i < nums.length; i++) ns[i] = new N(nums[i], i);
        Arrays.sort(ns, new Comparator<N>() {
            public int compare(N n1, N n2) {
                return Integer.compare(n1.v, n2.v);
            }
        });
        int l = 0, r = nums.length - 1;
        while(l < r) {
            if(ns[l].v + ns[r].v == target) return new int[] { ns[l].i, ns[r].i };
            else if(ns[l].v + ns[r].v < target) l++;
            else r--;
        }
        return null;
    }

    class N {
        int v;
        int i;
        public N(int v, int i) {
            this.v = v;
            this.i = i;
        }
    }