题解 | #四则运算#

#include <cctype>
#include <iostream>
#include <stack>
#include <string>
#include <bits/stdc++.h>
using namespace std;

bool compare_prioriy(char m, char n){
    if(((m == '*' || m == '/' || m == '+' || m == '-') && (n == '+' || n == '-')) || ((m == '*' || m == '/') && (n == '*' || n == '/'))) return true;
    else return false;
}

void compute(stack<float> &figure, stack<char> &operand){ 
    if(!figure.empty() && !operand.empty()){
        float y = figure.top();
        figure.pop();
        float x = figure.top();
        figure.pop();
        char ch = operand.top();
        operand.pop();
        int result;
        switch(ch){
            case '+': result = x + y; break;
            case '-': result = x - y; break;
            case '*': result = x * y; break;
            case '/': result = x / y; break;
            default: break;
        }
        figure.push(result);
    }
}

int main() {
    stack<float> figure;
    stack<char> operand;
    string str;
    cin >> str;
    bool flag = false;//标记是运算符还是正负号
    for(int i = 0 ; i < str.length(); i++){
        if(str[i] == '(' || str[i] == '[' || str[i] == '{'){
            operand.push('(');
        }
        else if(str[i] == ')' || str[i] == ']' || str[i] == '}'){
            while(!operand.empty() && operand.top() != '('){
                compute(figure, operand);
            }
            operand.pop();
        }
        else if(!flag){
            int j = i;
            if(str[j] == '+' || str[j] == '-'){
                i++;
            }
            while(isdigit(str[i])){//多位数的情况
                i++;
            }
            figure.push(stoi(str.substr(j, i-j)));
            i--;
            flag = true;//除括号外上一个字符是数字，则之后的+-会被当作运算符
        }
        else{
            //栈顶和当前字符都是运算符且栈顶运算符的优先级>=当前运算符的优先级时要先进行运算（连续两次减法或除法不能调换运算顺序）
            while(!operand.empty() && compare_prioriy(operand.top(), str[i])){
                compute(figure, operand);
            }
            operand.push(str[i]);
            flag = false;//除括号外上一个字符是运算符，则下一个+-是正负号
        }
    }
    //括号内算完之后栈里可能还有内容，从栈顶依次运算即可
    while(!operand.empty()){
        compute(figure, operand);
    }
    cout << figure.top() << endl;
    return 0;
}