指針指針：分區鏈接列表

Question

我試圖變得更容易與指針。所以，爲了好玩，我採取了以下C++函數，圍繞分隔值

void partitionList(lnode<int> *& head, int val) { 
    lnode<int> * front = nullptr; 
    lnode<int> * back = nullptr; 
    lnode<int> * curr = head; 

    while (curr) { 
     lnode<int> * next = curr->next; 
     if (curr->data < val) { 
      curr->next = front; 
      front = curr; 
     } else { 
      curr->next = back; 
      back = curr; 
     } 
     curr = next; 
    } 

    curr = front; 
    while (curr->next) { 
     curr = curr->next; 
    } 
    curr->next = back; 
    head = front; 
} 

一個鏈表，我試圖改變它採取C風格的雙指針來代替。我做了一個無意識的發現替換，這沒有奏效。尋找到它，我發現我的問題的根源，但我還是真的不明白這是怎麼回事...

void partitionList(lnode<int> ** head, int val) { 
    lnode<int> * front = nullptr; 
    lnode<int> * back = nullptr; 
    lnode<int> ** curr = head; 

    while (*curr) { 
     lnode<int> * entry = *curr; 

     std::cout << (*curr)->data << std::endl; // On second loop, prints 2 
     std::cout << entry->data << std::endl; // On second loop, prints 2 

     lnode<int> * next = entry->next; // This assignment does something 

     std::cout << entry->data << std::endl; // On second loop, prints 2 
     std::cout << (*curr)->data << std::endl; // On second loop, prints 3! 

     if ((*curr)->data < val) { 
      (*curr)->next = front; 
      front = *curr; 
     } else { 
      (*curr)->next = back; 
      back = *curr; 
     } 
     curr = &next; 
    } 

    *curr = front; 
    while ((*curr)->next) { 
     (*curr) = (*curr)->next; 
    } 
    (*curr)->next = back; 
    head = &front; 
} 

int main() { 
    lnode<int> * tail = new lnode<int>(8, nullptr); 
    lnode<int> * seven = new lnode<int>(7, tail); 
    lnode<int> * six = new lnode<int>(6, seven); 
    lnode<int> * five = new lnode<int>(5, six); 
    lnode<int> * four = new lnode<int>(4, five); 
    lnode<int> * three = new lnode<int>(3, four); 
    lnode<int> * two = new lnode<int>(2, three); 
    lnode<int> * head = new lnode<int>(1, two); 

    partitionList(&head, 6); 
} 

在第一循環中，調試打印線的所有四個附近的頂部函數的while循環打印「1」。但是，在第二個循環中，它們打印「2」，「2」，「2」，「3」？

任何人都可以解釋發生了什麼？什麼是使用雙指針而不是對指針的引用的正確方法？

謝謝！

Answer 1

void partitionList(lnode<int> *& head, int val) { 
    ... 
    head = front; // <= head is modified 
} 

但這裏不是

void partitionList(lnode<int> ** head, int val) { 
    ... 
    head = &front; // try it with *head = front 
} 

，如果你想快速更換此

void partitionList(lnode<int> *& head, int val) { 
    ... 
    lnode<int> * curr = head; 

只是寫：

void partitionList(lnode<int> ** head, int val) { 
    ... 
    lnode<int> * curr = *head; 

你爲什麼要修改*cur到**cur？