commit e1f560dc02af5400094a5d2aa161822242c7b029
Author: heimoshuiyu <heimoshuiyu@gmail.com>
Date:   Tue Mar 2 09:35:46 2021 +0800

    first commit

diff --git a/week1/task1.c b/week1/task1.c
new file mode 100644
index 0000000..57b9975
--- /dev/null
+++ b/week1/task1.c
@@ -0,0 +1,31 @@
+/*
+ * Author: Walter
+ * Student ID: 1930006025
+ * Week_1_Task_1
+ * Read in a positive number and compute its factorial using resursion
+ */
+
+#include <stdio.h>
+#include <assert.h>
+
+long int factR(int n);
+
+int main() {
+	int n;
+	printf("Enter a positive number to get its factorial\n");
+	scanf("%d", &n);
+	printf("Result is %ld\n", factR(n));
+	return 0;
+}
+
+long int factR(int n) {
+	/* check */
+	assert(n > 0);
+
+	/* end of recursion */
+	if (n == 1) {
+		return 1;
+	}
+
+	return n * factR(n-1);
+}
diff --git a/week1/task2.c b/week1/task2.c
new file mode 100644
index 0000000..be96566
--- /dev/null
+++ b/week1/task2.c
@@ -0,0 +1,41 @@
+/*
+ * Author: Walter
+ * Student ID: 1930006025
+ * Week_1_Task_2
+ * Read in and reverse a string using recursion
+ */
+
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+
+void reverseR(char * str, int length);
+
+int main() {
+	char str[50];
+	printf("Enter a string: ");
+	scanf("%s", str);
+	reverseR(str, strlen(str));
+	printf("Result: %s\n", str);
+	return 0;
+}
+
+void reverseR(char *str, int length) {
+	/* check */
+	assert(length <= 50);
+	assert(length >= 0);
+
+	/* end of recursion */
+	if (length <= 1) {
+		return;
+	}
+
+	/* recursion */
+	char tmp;
+	tmp = str[0];
+	str[0] = str[length-1];
+	str[length-1] = tmp;
+
+	/* str move forward, length decrese 2 char */
+	reverseR(str+1, length-2);
+}
diff --git a/week1/task3.c b/week1/task3.c
new file mode 100644
index 0000000..a414450
--- /dev/null
+++ b/week1/task3.c
@@ -0,0 +1,48 @@
+/*
+ * Author: Walter
+ * Student ID: 1930006025
+ * Week_1_Task_3
+ * Read in and compute the greatest common divisor (GCD) of
+ * two natural numbers using recursion
+ */
+
+#include <stdio.h>
+#include <assert.h>
+
+int GCD(int x, int y);
+
+/* the real GCD function */
+int _GCD(int x, int y);
+
+int main() {
+	int x, y;
+	printf("Please input x\n");
+	scanf("%d", &x);
+	printf("Please input y\n");
+	scanf("%d", &y);
+	printf("Result is %d\n", GCD(x, y));
+	return 0;
+}
+
+int GCD(int x, int y) {
+	/* check x y are natural numbers */
+	assert(x >= 0);
+	assert(y >= 0);
+
+	/* make sure x >= y,
+	 * then call the read GCD function */
+	if (x >= y) {
+		return _GCD(x, y);
+	} else {
+		return _GCD(y, x);
+	}
+}
+
+int _GCD(int x, int y) {
+	/* end of recursion */
+	if (y == 0) {
+		return x;
+	}
+
+	return _GCD(y, x % y);
+}