Added LC 494 java solution (#144)

Author: Tahanima

Algorithms/Medium/494_TargetSum/Solution.java

@@ -0,0 +1,35 @@
+class Solution {
+    int[] nums;
+    int n, target;
+    HashMap<ArrayList<Integer>, Integer> dp = new HashMap<>();
+
+    private int solve(int pos, int sum) {
+        ArrayList<Integer> aList = new ArrayList<>();
+        aList.add(pos);
+        aList.add(sum);
+
+        if (pos >= n) {
+            if (sum == target) {
+                return 1;
+            }
+
+            return 0;
+        } else if (dp.containsKey(aList)) {
+            return dp.get(aList);
+        } else {
+            int ans = solve(pos + 1, sum + nums[pos]);
+            ans += solve(pos + 1, sum - nums[pos]);
+            dp.put(aList, ans);
+
+            return ans;
+        }
+    }
+
+    public int findTargetSumWays(int[] nums, int target) {
+        this.nums = nums;
+        n = nums.length;
+        this.target = target;
+
+        return solve(1, nums[0]) + solve(1, -nums[0]);
+    }
+}