diff --git a/semester3/pset0/7-1-Balancing-Symbols.cpp b/semester3/pset0/7-1-Balancing-Symbols.cpp
new file mode 100644
index 0000000..4d71455
--- /dev/null
+++ b/semester3/pset0/7-1-Balancing-Symbols.cpp
@@ -0,0 +1,71 @@
+#include <cstdio>
+#include <iostream>
+#include <pthread.h>
+#include <stack>
+#include <string>
+#include <vector>
+
+char getParen(const char &ch) {
+  if (ch == ']') {
+    return '[';
+  } else if (ch == ')') {
+    return '(';
+  } else {
+    return '{';
+  }
+}
+
+int main(void) {
+  std::string input;
+  std::cin >> input;
+  std::stack<char> st;
+  bool ans[4] = {false, false, false, false};
+
+  for (char ch : input) {
+    // scan for errors
+    // () [] {}
+    if (ch == '(' || ch == '[' || ch == '{') {
+      st.push(ch);
+    } else if (ch == ')' || ch == ']' || ch == '}') {
+      // check
+      if (!st.empty() && st.top() == getParen(ch)) {
+        // pop, nothing wrong
+        st.pop();
+      } else {
+        if (ch == ')') {
+          ans[1] = true;
+        } else if (ch == ']') {
+          ans[2] = true;
+        } else {
+          ans[3] = true;
+        }
+        ans[0] = true;
+      }
+    }
+  }
+
+  while (!st.empty()) {
+    if (st.top() == '(') {
+      ans[1] = true;
+    } else if (st.top() == '[') {
+      ans[2] = true;
+    } else {
+      ans[3] = true;
+    }
+
+    ans[0] = true;
+    st.pop();
+  }
+
+  if (ans[0] == false) {
+    printf("0");
+  } else {
+    for (int i = 1; i < 4; i++) {
+      if (ans[i] == true) {
+        printf("%d,", i);
+      }
+    }
+  }
+
+  return 0;
+}
diff --git a/semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp b/semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp
new file mode 100644
index 0000000..23a5dad
--- /dev/null
+++ b/semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp
@@ -0,0 +1,92 @@
+// Convert the infix expression to postfix expression.
+#include <cctype>
+#include <cstdio>
+#include <iostream>
+#include <stack>
+#include <string>
+using namespace std;
+
+int prec(char ch) {
+  if (ch == '+' || ch == '-') {
+    return 0;
+  } else if (ch == '*' || ch == '/') {
+    return 1;
+  } else {
+    return -1;
+  }
+}
+
+void printEval(string &postfix) {
+  stack<double> st;
+  double l, r;
+  double ans;
+  for (char ch : postfix) {
+    if (isdigit(ch)) {
+      st.push(ch - '0');
+    } else {
+      r = st.top();
+      st.pop();
+      l = st.top();
+      st.pop();
+      if (ch == '+') {
+	ans = l + r;
+      } else if (ch == '-') {
+	ans = l - r;
+      } else if (ch == '*') {
+	ans = l * r;
+      } else if (ch == '/') {
+	ans = l / r;
+      }
+      st.push(ans);
+    }
+  }
+  printf("%.2lf\n", st.top());
+}
+
+void printFix(string &postfix) {
+  for (char ch : postfix) {
+    printf("%c ", ch);
+  }
+}
+
+int main(void) {
+  string input;
+  cin >> input;
+
+  stack<char> st;
+  string postfix;
+
+  for (char ch : input) {
+    if (isdigit(ch)) {
+      postfix.push_back(ch);
+    } else {
+      // if is not a digit
+      if (ch == '(') {
+        st.push(ch);
+      } else if (ch == ')') {
+        while (st.top() != '(') {
+          postfix.push_back(st.top());
+          st.pop();
+        }
+        st.pop();
+      } else {
+        // a operand
+        while (!st.empty() && prec(ch) <= prec(st.top())) {
+          postfix.push_back(st.top());
+          st.pop();
+        }
+        st.push(ch);
+      }
+    }
+  }
+
+  while (!st.empty()) {
+    postfix.push_back(st.top());
+    st.pop();
+  }
+
+  printEval(postfix);
+  printFix(postfix);
+
+  return 0;
+}
diff --git a/semester3/pset0/7-3-central-list.c b/semester3/pset0/7-3-central-list.c
new file mode 100644
index 0000000..a06776f
--- /dev/null
+++ b/semester3/pset0/7-3-central-list.c
@@ -0,0 +1,63 @@
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct stack {
+  int val;
+  struct stack *next;
+} stack;
+
+stack *push(stack *st, int val) {
+  stack *new = malloc(sizeof(stack));
+  new->next = st;
+  new->val = val;
+  return new;
+}
+
+int isEmpty(stack *st) { return st == NULL; }
+
+int peek(stack *st) { return st->val; }
+
+stack *pop(stack *st) {
+  stack *tmp = st->next;
+  free(st);
+  return tmp;
+}
+
+int main(void) {
+  stack *st = NULL;
+
+  int size = 0;
+  int in;
+  int last;
+  int lastUsed = 0;
+  int centFound = 0;
+
+  while (scanf("%d,", &in) != EOF) {
+    size++;
+    if (!isEmpty(st) && peek(st) == in) {
+      st = pop(st);
+    } else if (lastUsed == 1 && last == in) {
+      st = pop(st);
+      st = pop(st);
+      centFound = 1;
+    } else {
+      if (!isEmpty(st)) {
+        last = peek(st);
+        lastUsed = 1;
+      }
+      // printf("PUSHING: %d\n", in);
+      st = push(st, in);
+    }
+  }
+
+  if (size > 0 && isEmpty(st) && centFound == 1) {
+    printf("Yes");
+  } else if (size == 1) {
+    printf("Yes");
+  } else {
+    printf("No");
+  }
+
+  return 0;
+}
diff --git a/semester3/pset0/7-4-who-is-the-last.c b/semester3/pset0/7-4-who-is-the-last.c
new file mode 100644
index 0000000..abfa7ec
--- /dev/null
+++ b/semester3/pset0/7-4-who-is-the-last.c
@@ -0,0 +1,84 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct listNode {
+  int id;
+  struct listNode *next;
+  int out;
+} listNode;
+
+listNode *appendNode(int id, listNode *head) {
+  listNode *new = malloc(sizeof(listNode));
+  new->id = id;
+  new->next = NULL;
+  new->out = 0;
+
+  head->next = new;
+  return new;
+}
+
+listNode *makecircle(const int N) {
+  listNode *head = malloc(sizeof(listNode));
+  head->id = 1;
+  head->next = NULL;
+  head->out = 0;
+
+  listNode *ptr = head;
+  for (int i = 2; i <= N; i++) {
+    ptr = appendNode(i, ptr);
+    // printf("appending: %d\n", i);
+  }
+  ptr->next = head;
+
+  return head;
+}
+
+void printCircle(listNode *head) {
+  if (head->out == 0) {
+    printf("%d->", head->id);
+  }
+  head = head->next;
+  while (head->id != 1) {
+    if (head->out == 0) {
+      printf("%d->", head->id);
+    }
+    head = head->next;
+  }
+  printf("|");
+}
+
+int main(void) {
+  int N, M;
+  scanf("%d,%d", &N, &M);
+
+  listNode *head = makecircle(N);
+  // printCircle(head);
+
+  int count = N;
+  int step = 0;
+  listNode *ptr = head;
+  while (count != 0) {
+    while (step != M) {
+      while (ptr->out == 1) {
+        ptr = ptr->next;
+      }
+      ptr = ptr->next;
+      step++;
+    }
+    step = 0;
+    while (ptr->out == 1) {
+      ptr = ptr->next;
+    }
+
+    ptr->out = 1;
+    count--;
+
+    if (count != 0) {
+      printf("%d,", ptr->id);
+    }
+  }
+
+  printf("%d", ptr->id);
+
+  return 0;
+}