C语言链表头结点指针域不赋值出现错误？

Question

用单链表实现多项式的加法和乘法

很基础的一道数据结构题目，用C语言实现了

主要的函数和main函数如下

#include <stdio.h>
#include <stdlib.h>

//多项式结构体
typedef struct PolyNode *Polynomial;
struct PolyNode{
    int coef;//系数
    int expon;//指数
    Polynomial link;//指向后一个节点的指针域
};

Polynomial ReadPoly();//读多项式
void Attach(int c, int e, Polynomial *pRear);//构造多项式
Polynomial Mult(Polynomial P1, Polynomial P2);//多项式乘法
Polynomial Add(Polynomial P1, Polynomial P2);//多项式加法
void PrintPoly(Polynomial P);//输出多项式

int main(int argc, const char * argv[]) {
    // insert code here...
    Polynomial P1, P2, PP, PS;
    P1 = ReadPoly();//读取两个多项式
    P2 = ReadPoly();
    PP = Mult(P1, P2);
    PrintPoly(PP);
    PS = Add(P1, P2);
    PrintPoly(PS);
    return 0;
}

Polynomial ReadPoly(){
    Polynomial P, Rear, t;
    int c, e, N;
    scanf("%d", &N);
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    P->link = NULL;
    Rear = P;
    while (N--) {
        scanf("%d %d", &c, &e);
        Attach(c, e, &Rear);
    }
    t = P;
    P = P->link;
    free(t);
    return P;
}

//Attach函数要构建链表表示多项式
void Attach(int c, int e, Polynomial *pRear){
    Polynomial P;
    
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    P->coef = c;
    P->expon = e;
    P->link = (*pRear)->link;
    (*pRear)->link = P;
    *pRear = P;
}

//加法
Polynomial Add(Polynomial P1, Polynomial P2){
    Polynomial front, rear, t1, t2;
    int sum;
    t1 = P1;
    t2 = P2;
    front = (Polynomial)malloc(sizeof(struct PolyNode));
    front->link = NULL;
    rear = front;
    while (t1 && t2) {
        if (t1->expon == t2->expon) {
            sum = t1->coef + t2->coef;
            if(sum)
                Attach(sum, t1->expon, &rear);
            t1 = t1->link;
            t2 = t2->link;
        }
        else if (t1->expon > t2->expon){
            Attach(t1->coef, t1->expon, &rear);
            t1 = t1->link;
        }
        else if (t1->expon < t2->expon){
            Attach(t2->coef, t2->expon, &rear);
            t2 = t2->link;
        }
    }
    while (t1){
        Attach(t1->coef, t1->expon, &rear);
        t1 = t1->link;
    }
    while (t2){
        Attach(t2->coef, t2->expon, &rear);
        t2 = t2->link;
    }
    rear = front;
    front = front->link;
    free(rear);
    return front;
}

//乘法
Polynomial Mult(Polynomial P1, Polynomial P2){
    Polynomial P, Rear, t1, t2, t;
    int c, e;
    
    if (!P1 || !P2)
        return NULL;
    
    t1 = P1;
    t2 = P2;
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    
    P->link = NULL;//**为什么要让p指向空？**
    
    Rear = P;
    while (t2) {
        Attach(t1->coef*t2->coef, t1->expon+t2->expon, &Rear);
        t2 = t2->link;
    }
    t1 = t1->link;
    while (t1) {
        t2 = P2;
        Rear = P;
        while (t2) {
            c = t1->coef * t2->coef;
            e = t1->expon + t2->expon;
            while (Rear->link && Rear->link->expon > e)
                Rear = Rear->link;
            if (Rear->link && Rear->link->expon == e) {
                if (Rear->link->coef + c)
                    Rear->link->coef += c;
                else {
                    t = Rear->link;
                    Rear->link = t->link;
                    free(t);
                }
            }
            else {
                t = (Polynomial)malloc(sizeof(struct PolyNode));
                t->coef = c;
                t->expon = e;
                t->link = Rear->link;
                Rear->link = t;
                Rear = Rear->link;
            }
            t2 = t2->link;
        }
        t1 = t1->link;
    }
    Rear = P;
    P = P->link;
    free(Rear);
    return P;
}

问题在于我把乘法中的P->link = NULL删掉之后，整个算法的乘法和加法都出错了，请问这是为什么呢？

Polynomial P, Rear, t1, t2, t;
int c, e;

if (!P1 || !P2)
    return NULL;

t1 = P1;
t2 = P2;
P = (Polynomial)malloc(sizeof(struct PolyNode));

P->link = NULL;//**为什么要让p指向空？**

Rear = P;

Answer 1

如果没有这个置 NULL，相当于你的多项式链表很可能没有结束标记（因为 malloc 分配的空间并不会全部置 0）。所以在后来遍历链表的时候就访问到垃圾地址，导致程序崩溃。