cs: add «494. Target Sum»

URL:	https://leetcode.com/problems/target-sum/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/target-sum.cs

@@ -0,0 +1,28 @@
+using System.Linq;
+
+public class Solution {
+    public int FindTargetSumWays(int[] nums, int target) {
+        var total = nums.Sum();
+
+        var dp = new int[2 * total + 1];
+        dp[nums[0] + total] = 1;
+        dp[-nums[0] + total] += 1;
+
+        for (int i = 1; i < nums.Length; ++i) {
+            var next = new int[2 * total + 1];
+
+            for (int sum = -total; sum <= total; ++sum) {
+                if (dp[sum + total] <= 0) {
+                    continue;
+                }
+
+                next[sum + nums[i] + total] += dp[sum + total];
+                next[sum - nums[i] + total] += dp[sum + total];
+            }
+
+            dp = next;
+        }
+
+        return Math.Abs(target) > total ? 0 : dp[target + total];
+    }
+}