diff --git a/semester3/pset4/7-3-航空公司VIP客户查询.c b/semester3/pset4/7-3-航空公司VIP客户查询.c
index 47b2eaa..5414bbc 100644
--- a/semester3/pset4/7-3-航空公司VIP客户查询.c
+++ b/semester3/pset4/7-3-航空公司VIP客户查询.c
@@ -6,7 +6,7 @@
 #define IDLEN 19
 #define P 13
 
-typedef struct hashtable {
+typedef struct {
   int m;
   int p;
   char **key;
diff --git a/semester3/pset5/7-1-Insertion-sort.c b/semester3/pset5/7-1-Insertion-sort.c
new file mode 100644
index 0000000..1e944dd
--- /dev/null
+++ b/semester3/pset5/7-1-Insertion-sort.c
@@ -0,0 +1,41 @@
+#include <stdio.h>
+
+void print(int *arr, int N) {
+  for (int i = 0; i < N; i++) {
+    printf("%d,", arr[i]);
+  }
+  printf("\n");
+}
+
+void insertionSort(int *arr, int N) {
+  int key;
+  int j;
+  // i starts at 1.
+  for (int i = 1; i < N; i++) {
+    key = arr[i];
+    j = i;
+
+    while (j > 0 && arr[j - 1] > key) {
+      arr[j] = arr[j - 1];
+      j--;
+    }
+
+    arr[j] = key;
+    print(arr, N);
+  }
+}
+
+int main(void) {
+  int N;
+
+  scanf("%d", &N);
+  int arr[N];
+
+  for (int i = 0; i < N; i++) {
+    scanf("%d,", &arr[i]);
+  }
+
+  insertionSort(arr, N);
+
+  return 0;
+}
diff --git a/semester3/pset5/7-2-Shell-Sort.c b/semester3/pset5/7-2-Shell-Sort.c
new file mode 100644
index 0000000..1871b4a
--- /dev/null
+++ b/semester3/pset5/7-2-Shell-Sort.c
@@ -0,0 +1,42 @@
+#include <stdio.h>
+
+void print(int *arr, int N) {
+  for (int i = 0; i < N; i++) {
+    printf("%d,", arr[i]);
+  }
+  printf("\n");
+}
+
+void shellSort(int *arr, int N) {
+  int j, key;
+  for (int gap = N / 2; gap > 0; gap /= 2) {
+    // insertion sort, with gaps
+    for (int i = gap; i < N; i++) {
+      key = arr[i];
+      j = i;
+
+      // remember to compare with gaps
+      while (j >= gap && arr[j - gap] < key) {
+        arr[j] = arr[j - gap];
+        j -= gap;
+      }
+      arr[j] = key;
+    }
+    print(arr, N);
+  }
+}
+
+int main(void) {
+  int N;
+
+  scanf("%d", &N);
+  int arr[N];
+
+  for (int i = 0; i < N; i++) {
+    scanf("%d,", &arr[i]);
+  }
+
+  shellSort(arr, N);
+
+  return 0;
+}
diff --git a/semester3/pset5/7-3-Quick-Sort.c b/semester3/pset5/7-3-Quick-Sort.c
new file mode 100644
index 0000000..618b1b4
--- /dev/null
+++ b/semester3/pset5/7-3-Quick-Sort.c
@@ -0,0 +1,56 @@
+#include <stdio.h>
+#define N 10
+#define CUTOFF 3
+
+void print(int *arr) {
+  for (int i = 0; i < N; i++) {
+    printf("%d,", arr[i]);
+  }
+}
+
+void swap(int *arr, int l, int r) {
+  if (l != r) {
+    int tmp = arr[l];
+    arr[l] = arr[r];
+    arr[r] = tmp;
+  }
+}
+
+int partition(int *arr, int l, int r) {
+  // both edges included
+  int index = l;
+  int pivot = arr[l];
+  for (int i = l + 1; i <= r; i++) {
+    if (arr[i] < pivot) {
+      swap(arr, i, index);
+      index++;
+    }
+  }
+
+  swap(arr, index, l);
+  return index;
+}
+
+void quickSort(int *arr, int l, int r) {
+
+  int pi;
+  if (l < r) {
+    if (r - l < CUTOFF) {
+      printf("insert(%d,%d):", l, r - l + 1);
+    } else {
+      printf("Qsort(%d,%d):", l, r);
+    }
+
+    pi = partition(arr, l, r);
+
+    print(arr);
+
+    quickSort(arr, l, pi - 1);
+    quickSort(arr, pi + 1, r);
+  }
+}
+
+int main(void) {
+  int arr[] = {49, 38, 65, 97, 76, 13, 27, 50, 2, 8};
+  return 0;
+}
diff --git a/semester3/pset5/7-4-Selection-Sort.c b/semester3/pset5/7-4-Selection-Sort.c
new file mode 100644
index 0000000..999a937
--- /dev/null
+++ b/semester3/pset5/7-4-Selection-Sort.c
@@ -0,0 +1,43 @@
+#include <stdio.h>
+
+void swap(int *arr, int l, int r) {
+  int tmp = arr[l];
+  arr[l] = arr[r];
+  arr[r] = tmp;
+}
+
+void print(int *arr, int N) {
+  for (int i = 0; i < N; i++) {
+    printf("%d,", arr[i]);
+  }
+  printf("\n");
+}
+
+void selectionSort(int *arr, int N) {
+  for (int i = 0; i < N - 1; i++) {
+    int min = arr[i];
+    int minIndex = i;
+    for (int j = i + 1; j < N; j++) {
+      if (arr[j] < min) {
+        min = arr[j];
+        minIndex = j;
+      }
+    }
+    swap(arr, minIndex, i);
+    print(arr, N);
+  }
+}
+
+int main(void) {
+  int N;
+  scanf("%d", &N);
+
+  int arr[N];
+  for (int i = 0; i < N; i++) {
+    scanf("%d,", &arr[i]);
+  }
+
+  selectionSort(arr, N);
+
+  return 0;
+}
diff --git a/semester3/pset5/7-5-Radix-Sort.c b/semester3/pset5/7-5-Radix-Sort.c
new file mode 100644
index 0000000..3f67500
--- /dev/null
+++ b/semester3/pset5/7-5-Radix-Sort.c
@@ -0,0 +1,75 @@
+#include <stdio.h>
+#define SIZE 42
+
+int getDigit(int i, int pass) {
+  while (--pass && i) {
+    i = i / 10;
+  }
+  return i % 10;
+}
+
+void print(int *arr, int N) {
+  for (int i = 0; i < N; i++) {
+    printf("%d,", arr[i]);
+  }
+  printf("\n");
+}
+
+void radixSort(int *arr, int N, int pass) {
+  int bucket[10] = {0};
+  int output[N];
+
+  for (int i = 0; i < N; i++) {
+    bucket[getDigit(arr[i], pass)]++;
+  }
+
+  for (int i = 1; i < 10; i++) {
+    bucket[i] += bucket[i - 1];
+  }
+
+  for (int i = N - 1; i >= 0; i--) {
+    output[bucket[getDigit(arr[i], pass)] - 1] = arr[i];
+    // this is important, to avoid collision
+    bucket[getDigit(arr[i], pass)]--;
+  }
+
+  for (int i = 0; i < N; i++) {
+    arr[i] = output[i];
+  }
+}
+
+int getDigits(int i) {
+  // this is used to offset the string
+  int count = 0;
+  do {
+    i /= 10;
+    count++;
+  } while (i);
+
+  return count;
+}
+
+int main(void) {
+  int arr[SIZE];
+  int N = 0;
+  char buf[SIZE];
+  fgets(buf, SIZE, stdin);
+  int offset = 0;
+
+  while (sscanf(buf + offset, "%d,", &arr[N]) != EOF) {
+    // remember the comma!
+    offset += getDigits(arr[N]) + 1;
+    N++;
+  }
+
+  int stepPass;
+  scanf("%d", &stepPass);
+
+  for (int i = 1; i <= stepPass; i++) {
+    radixSort(arr, N, i);
+  }
+
+  // only output the last step
+  print(arr, N);
+  return 0;
+}