diff --git a/semester3/pset1/7-1-Balancing-Symbols.c b/semester3/pset1/7-1-Balancing-Symbols.c
new file mode 100644
index 0000000..aa42892
--- /dev/null
+++ b/semester3/pset1/7-1-Balancing-Symbols.c
@@ -0,0 +1,93 @@
+#include <stdio.h>
+#include <stdlib.h>
+#define SIZE 100
+
+int parenToInt(char ch) {
+  if (ch == '(') {
+    return 0;
+  } else if (ch == ')') {
+    return 1;
+  } else if (ch == '[') {
+    return 2;
+  } else if (ch == ']') {
+    return 3;
+  } else if (ch == '{') {
+    return 4;
+  } else if (ch == '}') {
+    return 5;
+  } else {
+    return -1;
+  }
+}
+
+typedef struct stack {
+  int top;
+  int capacity;
+  int *array;
+} stack;
+
+stack *newStack(int capacity) {
+  stack *st = malloc(sizeof(stack));
+  st->capacity = 50;
+  st->top = -1;
+  st->array = malloc(st->capacity * sizeof(int));
+
+  return st;
+}
+
+void push(stack *st, int val) {
+  st->top++;
+  st->array[st->top] = val;
+}
+
+int peek(stack *st) { return st->array[st->top]; }
+
+void pop(stack *st) { st->top--; }
+
+int isEmpty(stack *st) { return (st->top == -1); }
+
+int main(void) {
+  char input[SIZE];
+  fgets(input, SIZE, stdin);
+  int num;
+  int ans[3] = {};
+
+  stack *st = newStack(SIZE);
+
+  for (int i = 0; input[i] != '\0'; i++) {
+    num = parenToInt(input[i]);
+    if (num != -1) {
+      if (num % 2 == 0) {
+        // left paren , push to stack
+        push(st, num);
+      } else {
+        // right paren
+        if (!isEmpty(st) && peek(st) == num - 1) {
+          pop(st);
+        } else {
+          // a error
+          ans[num / 2] = 1;
+        }
+      }
+    }
+  }
+
+  while (!isEmpty(st)) {
+    num = peek(st);
+    ans[num / 2] = 1;
+    pop(st);
+  }
+
+  int incorrect = ans[0] | ans[1] | ans[2];
+  if (incorrect) {
+    for (int i = 0; i < 3; i++) {
+      if (ans[i]) {
+        printf("%d,", i + 1);
+      }
+    }
+  } else {
+    printf("0");
+  }
+
+  return 0;
+}
diff --git a/semester3/pset0/7-1-Balancing-Symbols.cpp b/semester3/pset1/7-1-Balancing-Symbols.cpp
similarity index 100%
rename from semester3/pset0/7-1-Balancing-Symbols.cpp
rename to semester3/pset1/7-1-Balancing-Symbols.cpp
diff --git a/semester3/pset1/7-2-Infix-to-Postfix-Conversion.c b/semester3/pset1/7-2-Infix-to-Postfix-Conversion.c
new file mode 100644
index 0000000..925e484
--- /dev/null
+++ b/semester3/pset1/7-2-Infix-to-Postfix-Conversion.c
@@ -0,0 +1,26 @@
+#include <stdio.h>
+#define SIZE 100
+
+typedef struct stack {
+  int top;
+  int capacity;
+  int* array;
+} stack;
+
+void push(stack* st, int val) {
+  st->array[++st->top] = val;
+}
+
+void pop(stack* st) {
+  st->top--;
+}
+
+int peek(stack* st) {
+  return st->array[st->top];
+}
+
+int main(void) {
+  char input[SIZE];
+  
+  fscanf(input, SIZE, stdin);
+}
diff --git a/semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp b/semester3/pset1/7-2-Infix-to-Postfix-Conversion.cpp
similarity index 92%
rename from semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp
rename to semester3/pset1/7-2-Infix-to-Postfix-Conversion.cpp
index 23a5dad..3de1f3d 100644
--- a/semester3/pset0/7-2-Infix-to-Postfix-Conversion.cpp
+++ b/semester3/pset1/7-2-Infix-to-Postfix-Conversion.cpp
@@ -12,7 +12,9 @@ int prec(char ch) {
   } else if (ch == '*' || ch == '/') {
     return 1;
   } else {
-    return -1;
+    // the precidence is the lowest, to make sure it stays in the stack.
+    // calculate stuff outside the paren last
+    return -1; 
   }
 }
 
diff --git a/semester3/pset0/7-3-central-list.c b/semester3/pset1/7-3-central-list.c
similarity index 100%
rename from semester3/pset0/7-3-central-list.c
rename to semester3/pset1/7-3-central-list.c
diff --git a/semester3/pset0/7-4-who-is-the-last.c b/semester3/pset1/7-4-who-is-the-last.c
similarity index 100%
rename from semester3/pset0/7-4-who-is-the-last.c
rename to semester3/pset1/7-4-who-is-the-last.c
diff --git a/semester3/pset2/2-1-Binary-Search-Tree.c b/semester3/pset2/2-1-Binary-Search-Tree.c
new file mode 100644
index 0000000..32fa115
--- /dev/null
+++ b/semester3/pset2/2-1-Binary-Search-Tree.c
@@ -0,0 +1,88 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct treeNode {
+  int val;
+  struct treeNode *l;
+  struct treeNode *r;
+} treeNode;
+
+treeNode *newNode(int val) {
+  treeNode *root = malloc(sizeof(treeNode));
+  root->val = val;
+  root->l = NULL;
+  root->r = NULL;
+  return root;
+}
+
+void insertTree(treeNode *root, int val) {
+  // Only insert at leaf.
+  treeNode *par;
+  int isL;
+  treeNode *ptr = root;
+  while (ptr != NULL) {
+    par = ptr;
+    if (ptr->val < val) {
+      ptr = ptr->r;
+      isL = 0;
+    } else {
+      ptr = ptr->l;
+      isL = 1;
+    }
+  }
+
+  if (isL) {
+    par->l = newNode(val);
+  } else {
+    par->r = newNode(val);
+  }
+}
+
+void freeTree(treeNode *root) {
+  if (root) {
+    freeTree(root->l);
+    freeTree(root->r);
+    free(root);
+  }
+}
+
+void printLevel(treeNode *root, int level, int *hasOutput) {
+  if (root == NULL) {
+    return;
+  }
+  if (level == 1) {
+    *hasOutput = 1;
+    printf("%d,", root->val);
+  } else {
+    printLevel(root->l, level - 1, hasOutput);
+    printLevel(root->r, level - 1, hasOutput);
+  }
+}
+
+int main(void) {
+  int size;
+  int level;
+  scanf("%d", &size);
+
+  int tmp;
+  scanf("%d,", &tmp);
+  treeNode *root = newNode(tmp);
+
+  for (int i = 1; i < size; i++) {
+    scanf("%d,", &tmp);
+    insertTree(root, tmp);
+  }
+
+  scanf("%d", &level);
+  int *hasOutput = malloc(sizeof(int));
+  *hasOutput = 0;
+  printLevel(root, level, hasOutput);
+
+  if (*hasOutput == 0) {
+    printf("-1");
+  }
+
+  free(hasOutput);
+  freeTree(root);
+  return 0;
+}
diff --git a/semester3/pset2/2-2-Traverse-Binary-Search-Tree.c b/semester3/pset2/2-2-Traverse-Binary-Search-Tree.c
new file mode 100644
index 0000000..1229a78
--- /dev/null
+++ b/semester3/pset2/2-2-Traverse-Binary-Search-Tree.c
@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct treeNode {
+  int val;
+  struct treeNode *l;
+  struct treeNode *r;
+} treeNode;
+
+treeNode *newNode(int val) {
+  treeNode *root = malloc(sizeof(treeNode));
+  root->val = val;
+  root->l = NULL;
+  root->r = NULL;
+  return root;
+}
+
+void insertTree(treeNode *root, int val) {
+  // Only insert at leaf.
+  treeNode *par;
+  int isL;
+  treeNode *ptr = root;
+  while (ptr != NULL) {
+    par = ptr;
+    if (ptr->val < val) {
+      ptr = ptr->r;
+      isL = 0;
+    } else {
+      ptr = ptr->l;
+      isL = 1;
+    }
+  }
+
+  if (isL) {
+    par->l = newNode(val);
+  } else {
+    par->r = newNode(val);
+  }
+}
+
+void freeTree(treeNode *root) {
+  if (root) {
+    freeTree(root->l);
+    freeTree(root->r);
+    free(root);
+  }
+}
+
+void preorderTraversal(treeNode *root) {
+  if (root) {
+    printf("%d,", root->val);
+    preorderTraversal(root->l);
+    preorderTraversal(root->r);
+  }
+}
+
+int main(void) {
+  int tmp;
+  scanf("%d,", &tmp);
+  treeNode *root = newNode(tmp);
+
+  while (scanf("%d,", &tmp) != EOF) {
+    insertTree(root, tmp);
+  }
+
+  preorderTraversal(root);
+
+  freeTree(root);
+  return 0;
+}
diff --git a/semester3/pset2/2-3-AVL-Tree.c b/semester3/pset2/2-3-AVL-Tree.c
new file mode 100644
index 0000000..5a0602a
--- /dev/null
+++ b/semester3/pset2/2-3-AVL-Tree.c
@@ -0,0 +1,131 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct treeNode {
+  int val;
+  struct treeNode *l;
+  struct treeNode *r;
+  int height; // used to calculate balance factor
+} treeNode;
+
+int max(int a, int b) { return (a > b) ? a : b; }
+
+treeNode *newNode(int val) {
+  treeNode *root = malloc(sizeof(treeNode));
+  root->val = val;
+  root->l = NULL;
+  root->r = NULL;
+  root->height = 1; // update this manually
+  return root;
+}
+
+int height(treeNode *tr) {
+  // This function can be optimized.
+  if (!tr) {
+    return 0;
+  } else {
+    return 1 + max(height(tr->l), height(tr->r));
+  }
+}
+
+treeNode *leftRotate(treeNode *x) {
+  treeNode *y = x->r;
+  x->r = y->l;
+  y->l = x;
+
+  // Update height here.
+  x->height = height(x);
+  y->height = height(y);
+
+  return y;
+}
+
+treeNode *rightRotate(treeNode *y) {
+  treeNode *x = y->l;
+  y->l = x->r;
+  x->r = y;
+
+  x->height = height(x);
+  y->height = height(y);
+
+  return x;
+}
+
+int getBalance(treeNode *tr) {
+  if (!tr) {
+    return 0;
+  } else {
+    return height(tr->l) - height(tr->r);
+  }
+}
+
+treeNode *insertTree(treeNode *root, int val) {
+  // Use recursion.
+  if (!root) {
+    return newNode(val);
+  } else {
+    if (val > root->val) {
+      root->r = insertTree(root->r, val);
+    } else {
+      root->l = insertTree(root->l, val);
+    }
+
+    root->height = height(root);
+
+    int bal = getBalance(root);
+    // check the balance using balance and comparing val with childs
+    if (bal > 1) {
+      if (val < root->l->val) {
+        // left left
+        return rightRotate(root);
+      } else {
+        // left right
+        root->l = leftRotate(root->l);
+        return rightRotate(root);
+      }
+    } else if (bal < -1) {
+      if (val > root->r->val) {
+        // right right
+        return leftRotate(root);
+      } else {
+        // right left
+        root->r = rightRotate(root->r);
+        return leftRotate(root);
+      }
+    } else {
+      // the balance is correct, don't change anything
+      return root;
+    }
+  }
+}
+
+void freeTree(treeNode *root) {
+  if (root) {
+    freeTree(root->l);
+    freeTree(root->r);
+    free(root);
+  }
+}
+
+void preorderTraversal(treeNode *root) {
+  if (root) {
+    printf("%d,", root->val);
+    preorderTraversal(root->l);
+    preorderTraversal(root->r);
+  }
+}
+
+int main(void) {
+  int tmp;
+  treeNode *root = NULL;
+
+  while (scanf("%d,", &tmp) != EOF) {
+    // update every time, since root can be changed.
+    root = insertTree(root, tmp);
+  }
+
+  preorderTraversal(root);
+
+  freeTree(root);
+  return 0;
+}