算法训练 表达式计算

#include <iostream>#include <string>#include <sstream>#include <stack>#include <cctype>using namespace std;stack<int> stack_num;stack<char> stack_op;bool f(char ch1, char ch2){    // 1-2+3     1*2/3    if ((ch1 == '-'||ch1 == '+') && (ch2 == '+'|| ch2 == '-') )        return false;    if ((ch1 == '*' || ch1 == '/') && (ch2 == '*' || ch2 == '/'))        return false;    // 1*2-3    if ((ch1 == '*' || ch1 == '/') && (ch2 == '+' || ch2 == '-'))        return false;    // 1+2*3    if ((ch1 == '+' || ch1 == '-') && (ch2 == '*' || ch2 == '/'))        return true;}int calc(int num1, int num2, char op){    int ret = 0;    switch (op)    {    case '+': ret = num1 + num2; break;    case '-': ret = num1 - num2; break;    case '*': ret = num1 * num2; break;    case '/': ret = num1 / num2; break;    default:        break;    }    return ret;}// 5*((1-2)*(6-7+8*9-(34-12*1)))// 1-2+3*(4-5)int main(){    string str;    cin >> str;    for (unsigned int i = 0; i < str.length(); i++){        if (isdigit(str[i])){            string tmp = "";            stringstream ss;            int k;            while (isdigit(str[i])){                tmp += str[i];                i++;            }            i-=1;            ss << tmp;            ss >> k;            stack_num.push(k);        }else{            if (stack_op.empty() || str[i] == '('){                stack_op.push(str[i]);            }else{                if (str[i] == ')'){                   while (stack_op.top() != '(')                   {                       int num1, num2;                       if (!stack_num.empty()){                           num2 = stack_num.top();                           stack_num.pop();                       }else{                           num2 = 0;                       }                       if (!stack_num.empty()){                           num1 = stack_num.top();                           stack_num.pop();                       }else{                           num1 = 0;                       }                       char op = stack_op.top();                       stack_op.pop();                       stack_num.push(calc(num1, num2, op));                   }                   stack_op.pop(); // 除去stack_op 中的 (                }else{                    char ch1 = stack_op.top();                    char ch2 = str[i];                    if (f(ch1, ch2)){                        stack_op.push(str[i]);                    }else{                        int num2 = stack_num.top();                        stack_num.pop();                        int num1 = stack_num.top();                        stack_num.pop();                        int s = calc(num1, num2, stack_op.top());                        stack_op.pop();                        stack_num.push(s);                        stack_op.push(str[i]);                    } // end if (f(ch1,ch2))                }            }        }    }    // 清空尾数据    while (!stack_op.empty()){        int num1, num2;        if (!stack_num.empty()){            num2 = stack_num.top();            stack_num.pop();        }else{            num2 = 0;        }        if (!stack_num.empty()){            num1 = stack_num.top();            stack_num.pop();        }else{            num1 = 0;        }        char op = stack_op.top();        stack_op.pop();        stack_num.push(calc(num1, num2, op));    }    cout << stack_num.top() << endl;    return 0;}