[csharp] 494. Target Sum

Author: benjiwright

Diff for: csharp/494-Target-Sum.cs

@@ -0,0 +1,20 @@
+public class Solution
+{
+    public int FindTargetSumWays(int[] nums, int target)
+    {
+        return Dp(nums, target, nums.Length - 1, 0, new Dictionary<(int, int), int>());
+    }
+
+    private int Dp(int[] nums, int target, int index, int sum, Dictionary<(int, int), int> memo)
+    {
+        if (memo.ContainsKey((index, sum))) return memo[(index, sum)];
+        if (index < 0 && sum == target) return 1;
+        if (index < 0) return 0;
+
+        var positive = Dp(nums, target, index - 1, sum + nums[index], memo);
+        var negative = Dp(nums, target, index - 1, sum + -1 * nums[index], memo);
+
+        memo.Add((index, sum), positive + negative);
+        return memo[(index, sum)];
+    }
+}