题解 | #四则运算#

#include <iostream>
#include <stack>

using namespace std;

int priority(char op) {
    if (op == '#') return 0;
    if (op == '+' || op == '-') return 1;
    if (op == '*' || op == '/') return 2;
    else return -1;
}

int compute(int a, int b, char op) {
    switch (op) {
        case '+':
            return a + b;
        case '-':
            return a - b;
        case '*':
            return a * b;
        case '/':
            return b != 0 ? a / b : 0;
        default:
            return 0;
    }
}

int main() {
    string str;
    while (cin >> str) {
        stack<char> op;
        stack<int> num;
        string tem;
        char ch;
        str += '#';
        for (int i = 0; i < str.size(); ++i) {
            ch = i > 0 ? str[i - 1] : '$';
            if (str[i] >= '0' && str[i] <= '9') tem += str[i];
            else {
                if (str[i] == '[' || str[i] == '{') str[i] = '(';
                else if (str[i] == ']' || str[i] == '}') str[i] = ')';

                if (!(ch >= '0' && ch <= '9') && ch != ')' && str[i] == '-') {
                    tem += str[i];
                    continue;
                }
                if (!tem.empty()) {
                    num.push(stoi(tem));
                    tem.clear();
                }

                if (str[i] == '(') op.push(str[i]);
                else if (str[i] == ')') {
                    while (!op.empty() && op.top() != '(') {
                        char c = op.top();
                        op.pop();
                        int b = num.top();
                        num.pop();
                        int a = num.top();
                        num.pop();
                        int sum = compute(a, b, c);
                        num.push(sum);
                    }
                    op.pop();
                } else {
                    while (!op.empty() && priority(op.top()) >= priority(str[i])) {
                        char c = op.top();
                        op.pop();
                        int b = num.top();
                        num.pop();
                        int a = num.top();
                        num.pop();
                        int sum = compute(a, b, c);
                        num.push(sum);
                    }
                    op.push(str[i]);
                }
            }
        }
        cout << num.top() << endl;
    }
    return 0;
}