7-4 求前缀表达式的值 (25 分)

此问题的关键在于怎么存放待计算的数据（包括计算过程中的中间结果）以及怎样从输入字符串中分解出运算符和数据。

C语言版本： 

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
typedef struct Node {
	double Data[1000];
	int top;
}*Stack;
double Calculate(double a, double b, char c) {
	switch (c) {
	case '+': return a + b;
	case '-':return a - b;
	case '*':return a * b;
	case'/':return a / b;
	}
}
void Push(Stack S, double a) {
	S->Data[++S->top] = a;
}
double Pop(Stack S) {
	return S->Data[S->top--];
}
int IsEmpty(Stack S) {
	if (S->top == -1) return 1;
	return 0;
}

int main() {
	char s[1000];

	int len,i;
	double a, b, res;
	fgets(s, 1000,stdin);
	Stack S;
	S = (Stack)malloc(sizeof(struct Node));
	S->top = -1;
	len = strlen(s);
	for (i = len - 1; i >= 0; i--) {
		if (((s[i] - '0') >= 0) && ((s[i] - '0') <= 9)) {
			a = s[i] - '0';
			i--;
			while (s[i] != ' '&&i >= 0) {
				a = 10 * (s[i] - '0') + a;
				i--;
			}
			Push(S, a);
		}
		else if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/') {
				if (!IsEmpty(S)) {
					a = Pop(S);
				}
				else {
					printf("ERROR\n");
					return 0;
				}
				if (!IsEmpty(S)) b = Pop(S);
				else {
					printf("ERROR\n");
					return 0;
				}
				if (s[i] == '/'&&b == 0) {
					printf("ERROR\n");
					return 0;
				}
				res = Calculate(a, b, s[i]);
				Push(S, res);
		}
	}
	printf("%.1f", res);
	return 0;
}