由中缀表达式计算前缀表达式

算法思想：定义栈，从右向左扫描字符串，遇到运算符，则栈顶和次栈顶的元素出栈，计算结果入栈，重复上面的步骤，知道字符串扫描完毕。

#include <iostream>
#include <cstring>
#define MAXSIZE 256
using namespace std;
typedef struct stack
{
   int top;
   char stack[MAXSIZE];
}Stack;
void initStack(Stack *s)
{
     s->top=0;
}
 
int getPrefixValue(Stack *s1,char s[])
{
    int i=strlen(s);
    int temp1,temp2;
    int sum=0;
    char d;
    if(strlen(s)==0)
      {
            cout<<"中缀式不能为空."<<endl;
            return -1;
      }
     while(i>=0)
      {
           if(s[i-1]>='0'&&s[i-1]<='9')
            {
              s1->top++;
              s1->stack[s1->top]=s[i-1];
            }
           else
            {
              temp1=s1->stack[s1->top]-'0';
              s1->top--;
              temp2=s1->stack[s1->top]-'0';
              s1->top--;
              switch(s[i-1])
                  {
                     case '+':sum=temp1+temp2;break;
                     case '-':sum=temp1-temp2;break;
                     case '*':sum=temp1*temp2;break;
                     case '/':sum=temp1/temp2;break;
                  }
              s1->top++;
              s1->stack[s1->top]=sum+'0';
            }
          i--;
      }
      d=s1->stack[s1->top];
      sum=d-'0';
      return sum;
}
 
int main()
{
   Stack s1;
   char s[]="+-+7*345/62";
   int sum=0;
   initStack(&s1);
   cout<<"前缀式为:"<<s<<endl;
   sum=getPrefixValue(&s1,s);
   cout<<"结果求和为："<<sum<<endl;
    return 0;
}