diff --git a/thu_dsa/chp4/stackApp.cpp b/thu_dsa/chp4/stackApp.cpp
new file mode 100644
index 0000000..c892cea
--- /dev/null
+++ b/thu_dsa/chp4/stackApp.cpp
@@ -0,0 +1,55 @@
+#include "../chp2/Fib.h"
+#include "stackApp.h"
+
+/*
+* @brief : convert a decimal digit n to any base(2 <= base <= 36)
+* @args  :
+* @return: return the result in the form of char*
+*/
+char* convert(__int64 n, int base){
+	Stack<char> charStack;
+	while(n != 0){
+		charStack.push(digits[n % base]);
+		n /= base;
+	}
+	char* res = new char[charStack.getSize() + 1];
+	int pos = 0;
+	while (!charStack.empty()) res[pos++] = charStack.pop();
+	res[pos] = '\0';
+	return res;
+}
+
+/*
+* @brief : to judge if an expression of matches
+* @args  : exp
+* @return: return true if matches
+*/
+bool paren(const char exp[]){
+	Stack<char> parenStack;
+	char currParen, popped;
+	for(int ix = 0; (currParen = exp[ix]) != '\0'; ++ix){
+		switch(currParen){
+		case '(':
+		case '[':
+		case '{':
+			parenStack.push(currParen);
+			break;
+
+		case ')':
+			if (parenStack.empty()) return false;
+			if (parenStack.pop() != '(') return false;
+			break;
+
+		case ']':
+			if (parenStack.empty()) return false;
+			if (parenStack.pop() != '[') return false;
+			break;
+
+		case '}':
+			if (parenStack.empty()) return false;
+			if (parenStack.pop() != '{') return false;
+			break;
+		}
+	}
+	return parenStack.empty()? true : false;
+}
diff --git a/thu_dsa/chp4/stackApp.h b/thu_dsa/chp4/stackApp.h
new file mode 100644
index 0000000..427e644
--- /dev/null
+++ b/thu_dsa/chp4/stackApp.h
@@ -0,0 +1,24 @@
+#ifndef STACKAPP_H_
+#define STACKAPP_H_
+
+#include "Stack.h"
+
+static char digits[] = { '0','1','2','3','4','5','6','7','8','9',
+						'a','b','c','d','e','f','g','h','i','j',
+						'k','l','m','n','o','p','q','r','s','t',
+						'u','v','w','x','y','z' };
+/*
+ * @brief : convert a decimal digit n to any base(2 <= base <= 36)
+ * @args  : 
+ * @return: return the result in the form of char*
+ */
+char* convert(__int64 n, int base);
+
+/*
+* @brief : to judge if an expression of parens matches
+* @args  : exp
+* @return: return true if matches
+*/
+bool paren(const char exp[]);
+
+#endif
diff --git a/thu_dsa/chp4/testStack.cpp b/thu_dsa/chp4/testStack.cpp
index 6f70756..ffd3f9e 100644
--- a/thu_dsa/chp4/testStack.cpp
+++ b/thu_dsa/chp4/testStack.cpp
@@ -9,7 +9,7 @@
 void test_basic();
 void test_capacity();
 
-int main(){
+int stack_test_main(){
 	test_basic();
 	test_capacity();
 
diff --git a/thu_dsa/chp4/testStackApp.cpp b/thu_dsa/chp4/testStackApp.cpp
new file mode 100644
index 0000000..55b8228
--- /dev/null
+++ b/thu_dsa/chp4/testStackApp.cpp
@@ -0,0 +1,33 @@
+#include "../chp2/Fib.h"
+#include "stackApp.h"
+#include <iostream>
+#include <cassert>
+
+using std::cout;
+using std::endl;
+
+void test_convert();
+void test_paren();
+
+int main(){
+	cout << "Running tests......" << endl;
+	test_convert();
+	test_paren();
+
+	cout << "All tests passed." << endl;
+	system("pause");
+	return 0;
+}
+
+void test_convert(){
+	assert(strcmp(convert(1314, 16), "522") == 0);
+	assert(strcmp(convert(520, 2), "1000001000") == 0);
+	assert(strcmp(convert(520, 16), "208") == 0);
+}
+
+void test_paren(){
+	assert(paren("(())") == true);
+	assert(paren("()()") == true);
+	assert(paren("[(])") == false);
+	assert(paren("{[()]{[({})]}}") == true);
+}