一元多项式的加减乘法

输入一元多项式的参数，然后根据一元多项式加法减法乘法的计算法则，求解最终结果。用到了结构体，指针，操作的时候要注意。

不多说，上代码：

#include <stdio.h>
#include <malloc.h>

typedef struct node{
    float coef;
    int expn;
    struct node *next;
}PLOY;

void start(){
    printf("*******************************
");
    printf("两个一元多项式相加/相减,相乘：
");
    printf("*******************************
");
    printf("请选择操作：
");
    printf("0.退出
");
    printf("1.两个一元多项式相加
");
    printf("2.两个一元多项式相乘
");
    printf("3.两个一元多项式相减
"); 
} 

void insert(PLOY *head,PLOY  *inpt){
    PLOY *pre,*now;
    int signal=0;
    pre=head;//pre定义为现在的前一个链接
    if(pre->next==NULL){
        pre->next=inpt;
    } 
    else{
        now=pre->next;
        while(signal==0){
            if(inpt->expn<now->expn){
                if(now->next==NULL){
                    now->next=inpt;
                    signal=1;
                }
                else{
                    pre=now;
                    now=pre->next;
                }
            }
            else{
                if(inpt->expn > now->expn){
                    inpt->next=now;
                    pre->next=inpt;
                    signal=1;    
                }
                else{
                    now->coef=now->coef+inpt->coef;
                    signal=1;
                    free(inpt);
                    if(now->coef==0){
                        pre->next=now->next;
                        free(now); 
                    }
                }
            }
        }
    }
}

PLOY *creat(char ch){
    PLOY *head,*inpt;
    float x;
    int y;
    head=(PLOY *)malloc(sizeof(PLOY));
    head->next=NULL;
    printf("请输入一元多项式%c:(格式是系数 指数；以0 0结束！)
",ch);
    scanf("%f %d",&x,&y);
    while(x!=0){
        inpt=(PLOY *)malloc(sizeof(PLOY));
        inpt->coef=x;
        inpt->expn=y;
        inpt->next=NULL;
        insert(head,inpt);
        printf("请输入一元多项式%c的下一项:(以0 0结束！)
",ch);
        scanf("%f %d",&x,&y);
    }
    return head;
} 

PLOY *addPLOY(PLOY *head,PLOY *pre){
    PLOY *inpt;
    int flag=0;
    while(flag==0){
        if(pre->next==NULL)
            flag=1;
        else{
            pre=pre->next;
            inpt=(PLOY *)malloc(sizeof(PLOY));
            inpt->coef=pre->coef;
            inpt->expn=pre->expn;  
            inpt->next=NULL;
            insert(head,inpt);
        }
    }
    return head;
}

PLOY *minusPLOY(PLOY *head,PLOY *pre){
    PLOY *inpt;
    int flag=0;
    while(flag==0){
        if(pre->next==NULL){
            flag=1;
        }
        else{
            pre=pre->next;
            inpt=(PLOY *)malloc(sizeof(PLOY));
            inpt->coef=0-pre->coef;
            inpt->expn=pre->expn;
            inpt->next=NULL;
            insert(head,inpt);
        }
    }
    return head;
}

PLOY *byPLOY(PLOY *headl,PLOY *head2){
    PLOY *inpt,*res,*pre;
    int flag=0;
    res=(PLOY *)malloc(sizeof(PLOY));
    res->next=NULL;
    headl=headl->next;
    pre=head2;
    while(flag==0){
        if(pre->next==NULL){
            pre=head2;
            headl=headl->next;
            continue;
        }
        if(headl==NULL){
            flag=1;
            continue;
        }
        pre=pre->next;
        inpt=(PLOY *)malloc(sizeof(PLOY));
        inpt->coef=pre->coef * headl->coef;
        inpt->expn=pre->expn + headl->expn;
        inpt->next=NULL;
        insert(res,inpt);
    } 
    return res;
}

void print(PLOY *fun){
    PLOY *printing;
    int flag=0;
    printing=fun->next;
    if(fun->next==NULL){
        printf("0
");
        return ;
    }
    while(flag==0){
        if(printing->coef>0&&fun->next!=printing)
            printf("+");
        if(printing->coef==1);
        else if(printing->coef==-1)
            printf("-");
        else
            printf("%f",printing->coef);
        if(printing->expn!=0)
            printf("x^%d",printing->expn);
        else if((printing->coef==1)||(printing->coef==-1))
            printf("1");
        if(printing->next==NULL)
            flag=1;
        else
            printing=printing->next;
    }
    printf("
");
}

int main(){
    PLOY *f,*g;
    int sign=-1;
    start();
    while(sign!=0){
        scanf("%d",&sign);
        switch(sign){
            case 0:
                break;
            case 1:
                {
                    printf("你选择的操作是多项式相加：
");
                    f=creat('f');               //输入多项式f(x) 
                    printf("f(x)=");
                    print(f);
                    g=creat('g');               //输入多项式g(x) 
                    printf("g(x)=");
                    print(g);
                    printf("F(x)=f(x)+g(x)=");
                    f=addPLOY(f,g);
                    print(f);
                    sign=-1;
                    start();
                    break;
                }
            case 2:
                {
                    printf("你选择的操作是多项式相乘：
");
                    f=creat('f');
                    printf("f(x)=");
                    print(f);
                    g=creat('g');
                    printf("g(x)=");
                    print(g);
                    printf("F(x)=f(x)*g(x)=");
                    f=byPLOY(f,g);
                    print(f);
                    sign=-1;
                    start();
                    break;    
                }
            case 3:
                {
                    printf("你选择的操作是多项式相减：
");
                    f=creat('f');
                    printf("f(x)=");
                    print(f);
                    g=creat('g');
                    printf("g(x)=");
                    print(g);
                    printf("F(x)=f(x)-g(x)=");
                    f=minusPLOY(f,g);
                    print(f);
                    sign=-1;
                    start();
                    break;
                } 
            default:
                {
                    printf("输入有误！请重新选择操作！
");
                    start();
                    break;
                } 
        }
        
    }
}