[csharp] 46. Permutations

csharp/46-Permutations.cs

@@ -0,0 +1,26 @@
+public class Solution
+{
+    public IList<IList<int>> Permute(int[] nums)
+    {
+        var result = new List<IList<int>>();
+        PermuteRecurse(result, nums, 0);
+        return result;
+    }
+
+    private void PermuteRecurse(List<IList<int>> res, int[] arr, int start)
+    {
+        if (start == arr.Length)
+        {
+            var list = arr.Select(t => (t)).ToList();
+            res.Add(list);
+            return;
+        }
+
+        for (var i = start; i < arr.Length; i++)
+        {
+            (arr[start], arr[i]) = (arr[i], arr[start]);
+            PermuteRecurse(res, arr, start + 1);
+            (arr[start], arr[i]) = (arr[i], arr[start]);
+        }
+    }
+}

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)];
+    }
+}