cs: add «1372. Longest ZigZag Path in a Binary Tree»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/longest-zigzag-path-in-a-binary-tree.cs

@@ -0,0 +1,41 @@
+public class Solution {
+    public enum Direction {
+        Left,
+        Right,
+    }
+
+    private static Direction Next(Direction d) => d switch {
+        Direction.Left => Direction.Right,
+        Direction.Right => Direction.Left,
+        _ => throw new ArgumentException("Invalid direction")
+    };
+
+    private static TreeNode? Get(Direction d, TreeNode n) => d switch {
+        Direction.Left => n.left,
+        Direction.Right => n.right,
+        _ => throw new ArgumentException("Invalid direction")
+    };
+
+    private int LongestZigZag(TreeNode node, int steps, Direction direction) {
+        if (node == null) {
+            return steps;
+        }
+
+        // Current path
+        var maxSteps = steps;
+
+        // Switching direction
+        maxSteps = Math.Max(maxSteps, LongestZigZag(Get(direction, node), steps + 1, Next(direction)));
+
+        // Starting new path
+        maxSteps = Math.Max(maxSteps, LongestZigZag(Get(Next(direction), node), 0, direction));
+
+        return maxSteps;
+    }
+
+    public int LongestZigZag(TreeNode root)
+        => Math.Max(
+            LongestZigZag(root, -1, Direction.Left),
+            LongestZigZag(root, -1, Direction.Right)
+        );
+}