插入已排序的链表段错误

Question

It looks like in "SortedInsert", the head is always zero and then the code segfaults anyway... really frustrating. Any idea why the head is always zero even though I set it to something, and then why the code segfaults in general? Thanks

#include <iostream>
#include <cassert>
#include <string>
#include <stdlib.h>
#include <sstream>
using namespace std;

struct Node {
    Node* next = 0;
    int data;
    ~Node(){
        if (next != 0){
            delete next;
        }
    }
};

void SortedInsert(Node* head, int value){
    if(head == 0){
        Node* header = new Node;
        header->data = value;
        head = header;
        return;
    }
    cout << "TEST" << endl;
    Node* temp = head;
    while(temp != 0){
        if(value > temp->data){
            Node* insert = temp->next;
            Node* otherTemp = new Node;
            otherTemp->data = value;
            temp->next= otherTemp;
            temp->next->next = insert;
        }
    temp=temp->next;
    }
    return; 
    }

int main() {
   srand(32);
   Node* sortedList = 0;
   for (int i = 0; i < 10; i++){
       SortedInsert(sortedList, rand() % 100);
   }

   Node* temp = sortedList;
   for (int i=0; i < 9; i++){
       assert(temp->data <= temp->next->data);
       temp = temp->next;
   }

   delete sortedList;
}

Answer 1

SortedInsert has its own copy of the head pointer. When you change head inside the function it doesn't affect the value in main. The solution is to pass head by reference or by passing the address.

void SortedInsert(Node** head, int value) {
    //Use *head to refer to the head of the list
}
int main() {
    ...
    Node* sortedList = 0;
    SortedInsert(&sortedList, ...);
    ...
}

Or

void SortedInsert(Node*& head, int value) {
    //Use head to refer to the head of the list
}
int main() {
    ...
    Node* sortedList = 0;
    SortedInsert(sortedList, ...);
    ...
}

Answer 2

Try the following

void SortedInsert( Node* &head, int value )
{
    if ( head == nullptr || value < head->data )
    {
        head = new Node { head, value };
    }
    else
    {
        Node *current = head;

        while ( current->next != nullptr && !( value < current->next->data ) )
        {
            current = current->next;
        }

        Node *tmp = new Node { current->next, value };
        current->next = tmp;
    }
}

As for your funcion implementation then the function deals with a copy of the head. Any changes of the copy do not influence on the argument itself. You should pass the head by reference or return the head from the function.