feat(codes/c): update linkedlist_stack.c some code

codes/c/chapter_stack_and_queue/linkedlist_stack.c

@@ -14,27 +14,64 @@ struct LinkedListStack {
 typedef struct LinkedListStack LinkedListStack;
 
 void new(LinkedListStack* stk) {
+    // 创建头结点
+    stk->stkTop = (ListNode *)malloc(sizeof(ListNode));
+    stk->stkTop->next = NULL;
+    stk->stkTop->val = 0;
 
+    // 初始化栈大小
+    stk->stkSize = 0;
 }
 
 size_t size(LinkedListStack* stk) {
-
+    assert(stk);
+    return stk->stkSize;
 }
 
 bool empty(LinkedListStack* stk) {
-
+    assert(stk);
+    return size(stk) == 0;
 }
 
 void push(LinkedListStack* stk, int num) {
-
+    assert(stk);
+
+    // 创建一个新结点
+    ListNode *n = (ListNode *)malloc(sizeof(ListNode));
+    ListNode *h;
+    n->next = NULL;
+    n->val = num;
+
+    // 遍历链表，将新结点挂在最后面
+    h = stk->stkTop;
+    while(h && h->next) {
+         h = h->next;
+    }
+    h->next = n;
+
+    // 栈大小自增
+    stk->stkSize++;
 }
 
 void pop(LinkedListStack* stk) {
-
+    assert(stk);
+    ListNode *h = stk->stkTop;
+    ListNode *n;
+    // 找到倒数第一个结点 h -> (h->next) -> (h->next->next)
+    while(h && h->next && h->next->next) {
+         h = h->next;
+    }
+
+    // 先保存倒数个结点，断开
+    n = h->next;
+    h->next = NULL;
+
+    // 删除保存的结点
+    free(n);
 }
 
 int top(LinkedListStack* stk) {
-
+    assert(stk);
 }
 
 int main() {