代码

#include <algorithm>
#include <cstring>
#include <iostream>
#include <stack>
#include <unordered_map>

using namespace std;

stack<int> num;
stack<char> op;

void eval()
{
    auto b = num.top();
    num.pop();
    auto a = num.top();
    num.pop();
    auto c = op.top();
    op.pop();
    int x;
    if (c == '+')
        x = a + b;
    else if (c == '-')
        x = a - b;
    else if (c == '*')
        x = a * b;
    else
        x = a / b;
    num.push(x);
}

int main()
{
    // 运算符优先级
    unordered_map<char, int> pr{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};
    string str;
    bool needZero = true; // 适用于 -号的不同含义
    cin >> str;
    for (int i = 0; i < str.size(); i++) {
        auto c = str[i];
        if (isdigit(c)) {
            int x = 0, j = i;
            while (j < str.size() && isdigit(str[j]))
                x = x * 10 + str[j++] - '0';
            i = j - 1;
            num.push(x);
        }
        else if (c == '(')
            op.push(c);
        else if (c == ')') {
            while (op.top() != '(')
                eval();
            op.pop();
        }
        else {
            while (op.size() && op.top() != '(' && pr[op.top()] >= pr[c]) eval();
            if (c == '-') {
                if (needZero) num.push(0);
            }
            op.push(c);
        }
        if (c == '(') needZero = true;
        else needZero = false;
    }
    while (op.size())
        eval();
    cout << num.top() << endl;
    return 0;
}