爲什麼這個函數沒有成功地將一個節點插入到鏈表中？

Question

該函數應該將節點插入鏈表，但它有一個錯誤，插入的節點不會顯示在鏈接列表中。錯誤在哪裏？

int insert(struct Node *headList, int payload) { 
    struct Node *newNode; 
    newNode = malloc(sizeof(struct Node)); 
    assert (newNode != NULL); 
    newNode->payload = payload; 
    newNode->next = headList; 
    headList = newNode; 
    return 0; 
} 

我敢肯定，Node *headList是按值傳遞，因爲headList沒有這個功能之外改變，但我不知道如何解決這個問題。我需要一個指向某個指針的指針嗎？

Answer 1

此函數修改struct Node，這個指針指向：

int insert(struct Node *headList, int payload) { 
    ... 
    headList = newNode; 
    ... 
} 

但是修改指針本身，你需要傳遞指針的地址初始化指針的指針：

int insert(struct Node **headList, int payload) { 
    ... 
    *headList = newNode; // <-- modifies the pointer itself 
    ... 
} 

或者你也可以使用返回值，因爲return 0似乎有點沒用處理這個問題。

Answer 2

我沒有看到任何使用return語句所以不是用指針的指針去，返回更新headList指針呼叫者

struct Node * insert(struct Node *headList, int payload) { 
    struct Node *newNode; 
    newNode = malloc(sizeof(struct Node)); 
    assert(newNode != NULL); 
    newNode->payload = payload; 
    newNode->next = headList; 
    headList = newNode; 
    return headList; 
} 

//in caller function 
    //Some code 
    headList = insert(headList,100); 

如果你是非常具體的return語句指針的指針走

int insert(struct Node **headList, int payload) { 
    struct Node *newNode; 
    newNode = malloc(sizeof(struct Node)); 
    assert (newNode != NULL); 
    newNode->payload = payload; 
    newNode->next = headList; 
    *headList = newNode; 
    return 0; 
} 

//in caller function 
//Some code 
status = insert(&headList,100);