diff --git a/semester3/pset6/7-1-朋友圈点赞.c b/semester3/pset6/7-1-朋友圈点赞.c
new file mode 100644
index 0000000..89042a5
--- /dev/null
+++ b/semester3/pset6/7-1-朋友圈点赞.c
@@ -0,0 +1,59 @@
+#include <stdio.h>
+#define MAXLEN 256
+#define SIZE 5
+
+int getDigits(int i) {
+  int count = 0;
+  do {
+    i /= 10;
+    count++;
+  } while (i);
+
+  return count;
+}
+
+int main(void) {
+  // remember to initialize to 0
+  int matrix[SIZE][SIZE] = {};
+  int liked[SIZE][SIZE] = {};
+
+  char buf[MAXLEN];
+  fgets(buf, MAXLEN, stdin);
+  int offset = 0;
+  int src, dist;
+
+  while (sscanf(buf + offset, "%d,%d,", &src, &dist) != EOF) {
+    // printf("%d -> %d\n", src, dist);
+    offset += getDigits(src) + getDigits(dist) + 2;
+    matrix[src][dist] = 1;
+  }
+
+  // calculate likes
+  for (int i = 0; i < SIZE; i++) {
+    int count = 0;
+    for (int j = 0; j < SIZE; j++) {
+      if (matrix[i][j] == 1) {
+        if (++count % 2 == 1) {
+          // printf("%d liked %d, count: %d\n", i, j, count);
+          liked[i][j] = 1;
+        }
+      }
+    }
+  }
+
+  scanf("%d", &dist);
+
+  int hasLikes = 0;
+  for (int i = 0; i < SIZE; i++) {
+    if (liked[i][dist] == 1) {
+      hasLikes = 1;
+      printf("%d,", i);
+    }
+  }
+
+  if (!hasLikes) {
+    printf("-1");
+  }
+
+  return 0;
+}
diff --git a/semester3/pset6/7-2-拓扑排序.c b/semester3/pset6/7-2-拓扑排序.c
new file mode 100644
index 0000000..6ab1151
--- /dev/null
+++ b/semester3/pset6/7-2-拓扑排序.c
@@ -0,0 +1,184 @@
+// NEEDS REWORKING
+#include <stdio.h>
+#include <stdlib.h>
+#define SIZE 11
+
+typedef struct listNode {
+  char ID;
+  struct listNode *next;
+} listNode;
+
+typedef struct queue {
+  listNode **arr;
+  int back;
+  int front;
+} queue;
+
+listNode *newNode(char ID) {
+  listNode *new = malloc(sizeof(listNode));
+  new->ID = ID;
+  new->next = NULL;
+
+  return new;
+}
+
+queue *newQueue() {
+  queue *q = malloc(sizeof(queue));
+  q->arr = malloc(sizeof(listNode *) * SIZE);
+  q->back = 0;
+  q->front = 0;
+  return q;
+}
+
+void appendNode(listNode *node, listNode *new) {
+  while (node->next != NULL) {
+    node = node->next;
+  }
+  node->next = new;
+}
+
+void push(listNode *li, queue *q) { q->arr[q->back++] = li; }
+
+int isEmpty(queue *q) { return (q->front == q->back); }
+
+listNode *getFront(queue *q) { return q->arr[q->front]; }
+
+void pop(queue *q) {
+  if (!isEmpty(q)) {
+    q->front++;
+  }
+}
+
+listNode **makeGraph() {
+
+  listNode **graph = malloc(sizeof(listNode *) * SIZE);
+
+  // initialize graph
+  int count = 0;
+  graph[count++] = newNode('S');
+  for (; count < 10; count++) {
+    graph[count] = newNode(count + 'A' - 1);
+  }
+  graph[count++] = newNode('T');
+
+  appendNode(graph[0], newNode('A'));
+  appendNode(graph[0], newNode('D'));
+  appendNode(graph[0], newNode('G'));
+
+  appendNode(graph[1], newNode('B'));
+  appendNode(graph[1], newNode('E'));
+
+  appendNode(graph[2], newNode('C'));
+
+  appendNode(graph[3], newNode('T'));
+
+  appendNode(graph[4], newNode('A'));
+  appendNode(graph[4], newNode('E'));
+
+  appendNode(graph[5], newNode('C'));
+  appendNode(graph[5], newNode('F'));
+  appendNode(graph[5], newNode('I'));
+
+  appendNode(graph[6], newNode('C'));
+  appendNode(graph[6], newNode('T'));
+
+  appendNode(graph[7], newNode('D'));
+  appendNode(graph[7], newNode('E'));
+  appendNode(graph[7], newNode('H'));
+
+  appendNode(graph[8], newNode('E'));
+  appendNode(graph[8], newNode('I'));
+
+  appendNode(graph[9], newNode('F'));
+  appendNode(graph[9], newNode('T'));
+
+  return graph;
+}
+
+int ch2digit(char ch) {
+  if (ch == 'S') {
+    return 0;
+  } else if (ch == 'T') {
+    return 10;
+  } else {
+    return ch - 'A' + 1;
+  }
+}
+
+int *getIndegree(listNode **g) {
+  int *inDegree = malloc(sizeof(int) * SIZE);
+  for (int i = 0; i < SIZE; i++) { // init to 0
+    inDegree[i] = 0;
+  }
+
+  for (int i = 0; i < SIZE; i++) {
+    listNode *ptr = g[i]->next;
+    while (ptr != NULL) {
+      inDegree[ch2digit(ptr->ID)]++;
+      ptr = ptr->next;
+    }
+  }
+
+  // for (int i = 0; i < SIZE; i++) {
+  //   printf("%c: %d\n", g[i]->ID, inDegree[i]);
+  // }
+
+  return inDegree;
+}
+
+void topoSort(listNode **g) {
+  queue *q = newQueue();
+  int visited[SIZE] = {};
+  int *inDegree = getIndegree(g);
+
+  for (int count = 0; count < SIZE; count++) {
+
+    for (int i = 0; i < SIZE; i++) {
+      if (inDegree[i] == 0 && visited[i] == 0) {
+        // printf("pushing: %c\n", g[i]->ID);
+        push(g[i], q);
+        visited[i] = 1;
+      }
+    }
+
+    listNode *tmp;
+    while (!isEmpty(q)) {
+      tmp = getFront(q);
+      pop(q);
+      printf("%c,", tmp->ID);
+
+      tmp = g[ch2digit(tmp->ID)]->next;
+      while (tmp != NULL) {
+        if (--inDegree[ch2digit(tmp->ID)] == 0 &&
+            visited[ch2digit(tmp->ID)] == 0) {
+          push(tmp, q);
+          visited[ch2digit(tmp->ID)] = 1;
+        }
+        tmp = tmp->next;
+      }
+    }
+  }
+}
+
+void deleteEdge(listNode **g, char src, char dist) {
+  listNode *ptr = g[ch2digit(src)];
+  while (ptr != NULL && ptr->next != NULL) {
+    if (ptr->next->ID == dist) {
+      ptr->next = ptr->next->next;
+    }
+    ptr = ptr->next;
+  }
+}
+
+int main(void) {
+  queue *q = newQueue();
+  listNode **g = makeGraph();
+
+  char src, dist;
+  scanf("%c,%c", &src, &dist);
+  deleteEdge(g, src, dist);
+
+  topoSort(g);
+
+  return 0;
+}
diff --git a/semester3/pset6/7-3-Shortest-Path.c b/semester3/pset6/7-3-Shortest-Path.c
new file mode 100644
index 0000000..09ebb36
--- /dev/null
+++ b/semester3/pset6/7-3-Shortest-Path.c
@@ -0,0 +1,122 @@
+#include <stdio.h>
+#include <stdlib.h>
+#define SIZE 7
+#define MAX 100
+
+typedef struct node {
+  int index,
+      dist; // for the starting node, dist means shortest distance from start to
+            // here, for child nodes, dist means dist(parent, this)
+  struct node *next;
+  struct node *parent;
+} node;
+
+node *newNode(int index, int dist) {
+  node *new = malloc(sizeof(node));
+  new->index = index;
+  new->dist = dist;
+  new->next = NULL;
+  new->parent = NULL;
+
+  return new;
+}
+
+void appendEdge(node *src, int index, int dist) {
+  node *end = newNode(index, dist);
+  node *ptr = src;
+  while (ptr->next != NULL) {
+    ptr = ptr->next;
+  }
+  ptr->next = end;
+}
+
+node **makeGraph() {
+  node **g = malloc(sizeof(node *) * SIZE);
+  for (int i = 0; i < SIZE; i++) {
+    g[i] = newNode(i, MAX);
+  }
+  appendEdge(g[0], 1, 2);
+  appendEdge(g[0], 3, 1);
+
+  appendEdge(g[1], 3, 3);
+  appendEdge(g[1], 4, 10);
+
+  appendEdge(g[2], 0, 4);
+  appendEdge(g[2], 5, 5);
+
+  appendEdge(g[3], 2, 2);
+  appendEdge(g[3], 4, 2);
+  appendEdge(g[3], 5, 8);
+  appendEdge(g[3], 6, 4);
+
+  appendEdge(g[4], 6, 6);
+
+  appendEdge(g[6], 5, 1);
+  return g;
+}
+
+int getMinDistance(node **graph, int visited[]) {
+  int min = MAX, minIndex;
+  for (int i = 0; i < SIZE; i++) {
+    if (graph[i]->dist < min && visited[i] == 0) {
+      min = graph[i]->dist;
+      minIndex = i;
+    }
+  }
+  return minIndex;
+}
+
+void dijkstra(node **graph, int start) {
+  int visited[SIZE] = {};
+  for (int i = 0; i < SIZE; i++) {
+    int s = getMinDistance(graph, visited);
+    // printf("found start: %d\n", s);
+
+    visited[s] = 1;
+
+    node *ptr = graph[s]->next;
+
+    // update distance of unvisited nodes
+    while (ptr) {
+      if (visited[ptr->index] == 0 &&
+          graph[ptr->index]->dist > ptr->dist + graph[s]->dist) {
+        graph[ptr->index]->dist = ptr->dist + graph[s]->dist;
+        graph[ptr->index]->parent = graph[s];
+        // printf("Updated: %d, dist: %d\n", ptr->index,
+        // graph[ptr->index]->dist);
+      }
+      ptr = ptr->next;
+    }
+  }
+}
+
+int main(void) {
+  node **graph = makeGraph();
+
+  int start, end;
+  scanf("%d,%d", &start, &end);
+  start--;
+  end--;
+
+  // init the start's dist as 0
+  graph[start]->dist = 0;
+  dijkstra(graph, start);
+
+  if (graph[end]->parent == NULL) {
+    printf("-1");
+  } else {
+    int route[MAX];
+    int count = 0;
+    node *ptr = graph[end];
+    while (ptr) {
+      route[count++] = ptr->index;
+      ptr = ptr->parent;
+    }
+
+    for (int i = count - 1; i >= 0; i--) {
+      printf("%d,", route[i] + 1);
+    }
+  }
+
+  return 0;
+}