cs: add «2707. Extra Characters in a String»

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

cs/extra-characters-in-a-string.cs

@@ -0,0 +1,50 @@
+public class Solution {
+    private class Node {
+        public Dictionary<char, Node> Next { get; set; } = new();
+        public bool Has { get; set; } = false;
+
+        public Node() { }
+    }
+
+    private static Node MakeTrie(string[] dictionary) {
+        var root = new Node();
+
+        foreach (var word in dictionary) {
+            var node = root;
+            foreach (var c in word) {
+                if (!node.Next.ContainsKey(c)) {
+                    node.Next[c] = new();
+                }
+                node = node.Next[c];
+            }
+            node.Has = true;
+        }
+
+        return root;
+    }
+
+    public int MinExtraChar(string s, string[] dictionary) {
+        var trie = MakeTrie(dictionary);
+
+        var length = s.Length;
+        var dp = new int[length + 1];
+
+        for (var start = length - 1; start >= 0; --start) {
+            dp[start] = 1 + dp[start + 1];
+
+            var node = trie;
+            for (var end = start; end < length; ++end) {
+                if (!node.Next.ContainsKey(s[end])) {
+                    break;
+                }
+
+                node = node.Next[s[end]];
+                if (node.Has) {
+                    dp[start] = Math.Min(dp[start], dp[end + 1]);
+                }
+            }
+        }
+
+        return dp[0];
+    }
+}