四则运算表达式可以用表达式树表达，如下图后序遍历

现给你一个字符串，代表一个后序遍历形式的四则运算表达式，请计算出表达式的结果：(只输出整数部分)
注：除法只保留整数部分；5/4= 1

输入： 一个字符串，代表一个四则运算表达式，输入保证合法一定有计算结果，操作数是合法的整数，运算符只包含±*/;

输出： 一个整数 ：范围 -2147483648 ~ 2147483647

样例： 9，3，5，-，2，*，+；
输出：5

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


#define BUF_LEN 50000

static int Calc(const char* input)
{
	int i = 0;
	int stack[BUF_LEN] = { 0 };
	int top = 0;
	char *savep = NULL;
	char* p = strtok_s(input, ",", &savep);
	while (p != NULL) {
		if (strcmp(p, "+") == 0) {
			stack[top - 2] = stack[top - 2] + stack[top - 1];
			stack[top - 1] = 0;
			top--;
			p = strtok_s(NULL, ",", &savep);
			continue;
		}

		if (strcmp(p, "-") == 0) {
			stack[top - 2] = stack[top - 2] - stack[top - 1];
			stack[top - 1] = 0;
			top--;
			p = strtok_s(NULL, ",", &savep);
			continue;
		}

		if (strcmp(p, "*") == 0) {
			stack[top - 2] = stack[top - 2] * stack[top - 1];
			stack[top - 1] = 0;
			top--;
			p = strtok_s(NULL, ",", &savep);
			continue;
		}

		if (strcmp(p, "/") == 0) {
			stack[top - 2] = stack[top - 2] / stack[top - 1];
			stack[top - 1] = 0;
			top--;
			p = strtok_s(NULL, ",", &savep);
			continue;
		}
		(void)sscanf_s(p, "%d", &stack[top++]);
		p = strtok_s(NULL,  ",", &savep);
	}
	return stack[0];
}

int main(void)
{
	static char buf[BUF_LEN];
	if (get_s(buf, sizeof(buf)) == NULL) {
		return -1;
	}

	int ret = Calc(buf);
	(void)printf("%d", ret);
	return 0;
}