diff --git a/semester3/pset4/7-1-霍夫曼解码-REVISION.c b/semester3/pset4/7-1-霍夫曼解码-REVISION.c
new file mode 100644
index 0000000..2ba2147
--- /dev/null
+++ b/semester3/pset4/7-1-霍夫曼解码-REVISION.c
@@ -0,0 +1,130 @@
+// 14:47 - 15:23
+#include <stdio.h>
+#include <stdlib.h>
+#define MAXSIZE 150
+
+typedef struct node {
+  char ch;
+  int freq;
+  struct node *l, *r;
+} node;
+
+typedef struct heap {
+  int top;
+  node **arr;
+} heap;
+
+node *newNode(char ch, int freq) {
+  node *tr = malloc(sizeof(node));
+  tr->l = tr->r = NULL;
+  tr->ch = ch;
+  tr->freq = freq;
+  return tr;
+}
+
+heap *newHeap() {
+  heap *h = malloc(sizeof(heap));
+  h->top = 0;
+  h->arr = malloc(sizeof(node *) * MAXSIZE);
+  return h;
+}
+
+int parent(int i) { return (i - 1) / 2; }
+
+int left(int i) { return i * 2 + 1; }
+
+int right(int i) { return i * 2 + 2; }
+
+void swap(node **l, node **r) {
+  node *tmp = *l;
+  *l = *r;
+  *r = tmp;
+}
+
+void heapify(heap *h, int i) {
+  // from up to down
+  int smallest = i;
+  int l = left(i), r = right(i);
+
+  if (l < h->top && h->arr[smallest]->freq > h->arr[l]->freq) {
+    smallest = l;
+  }
+  if (r < h->top && h->arr[smallest]->freq > h->arr[r]->freq) {
+    smallest = r;
+  }
+
+  if (smallest != i) {
+    swap(&h->arr[smallest], &h->arr[i]);
+    heapify(h, smallest);
+  }
+}
+
+void insert(heap *h, node *tr) {
+  int i = h->top++;
+  h->arr[i] = tr;
+
+  while (h->arr[i]->freq < h->arr[parent(i)]->freq) {
+    swap(&h->arr[i], &h->arr[parent(i)]);
+    i = parent(i);
+  }
+}
+
+node *extractMin(heap *h) {
+  node *res = h->arr[0];
+  h->arr[0] = h->arr[--h->top];
+
+  heapify(h, 0);
+  return res;
+}
+
+node *makeHuffmanTree(heap *h) {
+  node *l, *r;
+  node *tmp;
+  while (h->top > 1) {
+    l = extractMin(h);
+    r = extractMin(h);
+
+    tmp = newNode('#', l->freq + r->freq);
+    tmp->l = l;
+    tmp->r = r;
+
+    insert(h, tmp);
+  }
+  return extractMin(h);
+}
+
+void printCode(node *tr, char *buf) {
+  int i = 0;
+  while (buf[i] == '0' || buf[i] == '1') {
+    node *cur = tr;
+    while (cur->ch == '#') {
+      if (buf[i] == '0') {
+        cur = cur->l;
+      } else {
+        cur = cur->r;
+      }
+      i++;
+    }
+    printf("%c", cur->ch);
+  }
+}
+
+int main(void) {
+  heap *h = newHeap();
+  insert(h, newNode('a', 7));
+  insert(h, newNode('b', 19));
+  insert(h, newNode('c', 2));
+  insert(h, newNode('d', 6));
+  insert(h, newNode('e', 32));
+  insert(h, newNode('f', 3));
+  insert(h, newNode('g', 21));
+  insert(h, newNode('h', 10));
+
+  node *hTree = makeHuffmanTree(h);
+  char buf[MAXSIZE];
+  fgets(buf, MAXSIZE, stdin);
+
+  printCode(hTree, buf);
+
+  return 0;
+}
diff --git a/semester3/pset7/7-4-哥尼斯堡的“七桥问题”.c b/semester3/pset7/7-4-哥尼斯堡的“七桥问题”.c
new file mode 100644
index 0000000..6ff1dfc
--- /dev/null
+++ b/semester3/pset7/7-4-哥尼斯堡的“七桥问题”.c
@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+int main(void) {
+  return 0;
+}