后缀表达式变换为中缀表达式

详细说明：将由数字和四则运算符组成的后缀表达式变换为中缀表达式。输入的后缀表达式包含的运算符不超过15个。要求转换后的中缀表达式中不应出现不必要的括号。例如，整个表达式两端的括号要省略，不影响原计算顺序的括号要省略。

后缀式转中缀式   

#include<stdio.h>   

#include<string.h>   

char str[50][1000],si[1000],t[1000];   

long gra[1000];   

int main(){   

    long l,sn,i,j,k;   

    gets(si);   

    l=strlen(si);   

    sn=0;   

    i=0;j=0;   

    while (j<l){   

          while ((j<l)&&(si[j]!=' '))   

                j++;   

          if ((j==i+1)&&(si[i]=='+')){   

             sn--;   

             if (gra[sn-1]<0){   

                strcpy(t,str[sn-1]);   

                strcpy(str[sn-1],"(");   

                strcat(str[sn-1],t);   

                strcat(str[sn-1],")");   

             }   

             strcat(str[sn-1],"+");   

             if ((gra[sn]==1)||(gra[sn]==2)||(gra[sn]<0)){   

                strcpy(t,str[sn]);   

                strcpy(str[sn],"(");   

                strcat(str[sn],t);   

                strcat(str[sn],")");   

             }   

             strcat(str[sn-1],str[sn]);   

             gra[sn-1]=1;   

             gra[sn]=0;   

          }   

          else if ((j==i+1)&&(si[i]=='-')){   

               sn--;   

               if (gra[sn-1]<0){   

                  strcpy(t,str[sn-1]);   

                  strcpy(str[sn-1],"(");   

                  strcat(str[sn-1],t);   

                  strcat(str[sn-1],")");   

               }   

               strcat(str[sn-1],"-");   

               if ((gra[sn]==1)||(gra[sn]==2)||(gra[sn]<0)){   

                  strcpy(t,str[sn]);   

                  strcpy(str[sn],"(");   

                  strcat(str[sn],t);   

                  strcat(str[sn],")");   

               }   

               strcat(str[sn-1],str[sn]);   

               gra[sn-1]=2;   

               gra[sn]=0;   

          }   

          else if ((j==i+1)&&(si[i]=='*')){   

               sn--;   

               if ((gra[sn-1]==1)||(gra[sn-1]==2)||(gra[sn-1]<0)){   

                  strcpy(t,str[sn-1]);   

                  strcpy(str[sn-1],"(");   

                  strcat(str[sn-1],t);   

                  strcat(str[sn-1],")");   

               }   

               strcat(str[sn-1],"*");   

               if (gra[sn]!=0){   

                  strcpy(t,str[sn]);   

                  strcpy(str[sn],"(");   

                  strcat(str[sn],t);   

                  strcat(str[sn],")");   

               }   

               strcat(str[sn-1],str[sn]);   

               gra[sn-1]=3;   

               gra[sn]=0;   

          }   

          else if ((j==i+1)&&(si[i]=='/')){   

               sn--;   

               if ((gra[sn-1]==1)||(gra[sn-1]==2)||(gra[sn-1]<0)){   

                  strcpy(t,str[sn-1]);   

                  strcpy(str[sn-1],"(");   

                  strcat(str[sn-1],t);   

                  strcat(str[sn-1],")");   

               }   

               strcat(str[sn-1],"/");   

               if (gra[sn]!=0){   

                  strcpy(t,str[sn]);   

                  strcpy(str[sn],"(");   

                  strcat(str[sn],t);   

                  strcat(str[sn],")");   

               }   

               strcat(str[sn-1],str[sn]);   

               gra[sn-1]=4;   

               gra[sn]=0;   

          }   

          else{   

               for (k=i;k<j;k++)   

                   str[sn][k-i]=si[k];   

               str[sn][j-i]='\0';   

               if (str[sn][0]=='-')   

                  gra[sn]=-1;   

               else   

                   gra[sn]=0;   

               sn++;   

          }   

          while ((j<l)&&(si[j]==' '))   

                j++;   

          i=j;j=i;   

    }   

    printf("%s\n",str[0]);   

    scanf("%ld",&sn);   

    return 0;   

}