Add treap clearing

treap.hpp

@@ -89,6 +89,13 @@ public:
 		// Don't find index k, but k-th line -> +1
 		return find(root, k+1);
 	}
+	void clear() {
+		while(size() > 0) {
+			auto two = split(root, 0);
+			root = two.second;
+			delete two.first;
+		}
+	}
 
 	// Line insert
 	void insert(int k, string s) {