任意输入含加减乘除乘方开方的文本数学表达式文本，自动计算结果

#
#include <iostream>
#include <string>
#include <stack>
#include <sstream>
#include <cmath>

using namespace std;

int main()
{
  string s = "2+3*4-5^2";
  stringstream ss;

  stack<int> nums;
  stack<char> signs;

  int num;
  char sign;
  while (!s.empty())
  {
    if (isdigit(s.back()))
    {
      ss << s.back();
      s.pop_back();

      if (s.empty() || !isdigit(s.back()))
      {
        ss >> num;
        nums.push(num);
        ss.clear();
      }
    }
    else
    {
      if (s.back() == '^')
      {
        int num = nums.top();
        nums.pop();
        num = pow(num, 2);
        nums.push(num);
      }