简单的混合四则运算

#include<iostream>  
#include<string>  
#include<math.h>  
using namespace std;
class stack {
  int a[10];
public:
  bool init();
  bool pop(int *i);
  bool push(int i);
  static int index;
  int cal();
};
int stack::index = 0;
bool stack::init() { index = 0; return true; }
bool stack::pop(int *i) {
  if (index > 0) { *i = a[index - 1]; index--; return true; }
  else
    return false;
}
bool stack::push(int i) {
  if (index < 10) { a[index] = i; index++; return true; }
  else
    return false;
}
int stack::cal() {
  int
    i = 0, num = 0; for (; i < index; i++) { num = num + a[i] * pow(10, index - i - 1); }return num;
}

class cal {
public:
  stack buf;
  int num[20];
  char ch[20];
  string str;
  cal() { for (int i = 0; i < 20; i++) { num[i] = NULL; ch[i] = NULL; } }
  void solve(int i, int j);//分解输入字符窜  
  int resolve();  //求出一个不含括号的混合算术表达式的值  
  int check_ch();  //返回运算中第一个乘号除号的位置，否则返回-1  
};
int cal::check_ch() {
  int i, j;
  for (i = 0; ch[i] != NULL; i++);
  j = 0;
  while (ch[j] != NULL && ch[j] != '*'&&ch[j] != '/') { j++; }
  if (j == i) return -1;
  else return j;

}
void cal::solve(int i = 0, int j = 0) {
  buf.init();
  while (str[i] > 47 && str[i] < 58)
  {
    buf.push((int)str[i] - 48);
    i++;
  }
  if (buf.index != 0)
    num[j] = buf.cal();
  if (i < str.length()) { ch[j] = str[i]; i++; j++; solve(i, j); }
}

int cal::resolve() {
  solve(0, 0);
  while (check_ch() != -1) {
    if (ch[check_ch()] == '*') {
      num[check_ch()] = num[check_ch()] * num[check_ch() + 1];
      for (int j = check_ch(); num[j + 1] != NULL && j < 18; j++) { num[j + 1] = num[j + 2]; }
      for (j = check_ch(); ch[j] != NULL && j < 19; j++) { ch[j] = ch[j + 1]; }
    }
    if (ch[check_ch()] == '/') {
      num[check_ch()] = num[check_ch()] / num[check_ch() + 1];
      for (int j = check_ch(); num[j + 1] != NULL && j < 18; j++) { num[j + 1] = num[j + 2]; }
      for (j = check_ch(); ch[j] != NULL && j < 19; j++) { ch[j] = ch[j + 1]; }
    }
  }
  while (ch[0] != NULL) {
    if (ch[0] == '+') {
      num[0] = num[0] + num[1];
      for (int j = 0; num[j + 1] != NULL && j < 18; j++) { num[j + 1] = num[j + 2]; }
      for (j = 0; ch[j] != NULL && j < 19; j++) { ch[j] = ch[j + 1]; }
    }
    if (ch[0] == '-') {
      num[0] = num[0] - num[1];
      for (int j = 0; num[j + 1] != NULL && j < 18; j++) { num[j + 1] = num[j + 2]; }
      for (j = 0; ch[j] != NULL && j < 19; j++) { ch[j] = ch[j + 1]; }
    }
  }
  return num[0];

}

void main() {

L:cal a;

  cin >> a.str;

  cout << a.resolve() << endl;
  goto L;
}