在列表C ++中查找第一个和第二个元素

Question

这是我用C ++实现列表的程序。我输入元素直到0.程序显示第一个元素正确但第二个是错误的。我可能在第二个条件下犯错误

if (p -> next == first) {
    secondElement = first -> data;
}

。你能说出它有什么问题吗？感谢

#include "stdafx.h"
#include "iostream"
using namespace std;

struct Node {
    int data;
    Node *next;
};

int firstElement;
int secondElement;

int main()
{
    Node *first = 0;
    Node *p;

    cout << "Enter a list" << endl;
    int i;
    while (true) {

        cin >> i;
        if (i == 0) break;


        p = new Node;
        p -> data = i;


        p -> next = first;

        if (first == 0) {
            first = p;
            firstElement = first -> data;
        }

        if (p -> next == first) {
            secondElement = first -> data;
        }       

        first = p;

    }

    cout << "First element is: " << firstElement << endl;
    cout << "Second element is: " << secondElement << endl;
    cout << "List: ";

    p = first;
    while (p) {
        cout << p -> data << " ";
        p = p -> next;
    }
    cout << endl;

    return 0;
}

Answer 1

你可以这样做（我刚刚编辑了你的while循环）：

while (true) {

    cin >> i;
    if (i == 0) break;


    p = new Node;
    p -> data = i;
    p -> next = 0;

    if (first != 0 && first->next == 0)
        secondElement = p->data;

    p -> next = first;

    if (first == 0) {
        first = p;
        firstElement = first -> data;
    }

    first = p;

}

希望这是你想要实现的......

Answer 2

每次循环都会将元素的指针设置为第一个。

p -> next = first;

然后在检查第二个元素时，你要检查指针是否设置为第一个，它始终是。

 if (p -> next == first) // This is always true

您必须使用一些不同的检查来查看它是否是列表中的第二个条目，例如：

if (p->next && !p->next->next) // only true second time around
{
    secondElement = p -> data; // also note the change here
}

Answer 3

p -> next = first;
......
enter code here
 if (p -> next == first) {  //it's always true here

你应该有

 if (p -> next == 0) {