表达式求值——加减乘除乘方负数括号不匹配

https://www.acwing.com/problem/content/description/153/

负数的处理逻辑是：一旦认为一个符号是符号，就相当于在负号之前有一个0压入numstack。

#include <iostream>
#include <cctype>
#include <stack>
#include <string>
#include <unordered_map>
#include <cmath>

using namespace std;

unordered_map<char, int> pr;
stack<int> num;
stack<char> op;

void eval() {
    int b = num.top(); num.pop();
    int a = num.top(); num.pop();
    char c = op.top();  op.pop();
    int r;
    if(c == '+') r = a + b;
    if(c == '-') r = a - b;
    if(c == '*') r = a * b;
    if(c == '/') r = a / b;
    if(c == '^') r = pow(a, b);
    num.push(r);
}


int main() {
    string str;
    cin >> str;
    pr['('] = 0;
    pr['+'] = pr['-'] = 1;
    pr['*'] = pr['/'] = 2;
    pr['^'] = 3;
    pr[')'] = 4;
    string left;
    for(int i = 0; i <= str.size(); ++i) left += '(';
    str = left + str + ')';

    // cout << str << endl;



    for(int i = 0; i < str.size(); ++i) {
        char c = str[i];
        if(isdigit(c)) {
            int j = i, x = 0;
            while(j < str.size() && isdigit(str[j])) x = 10 * x + (str[j ++] - '0');
            i = j - 1;
            num.push(x);
        }
        else if(c == '(') op.push(c);
        else if(c == ')') {
            while(op.size() && op.top() != '(') eval();
            op.pop();
        }
        else {
            if(c == '-') {
                if((i == 0) || (i && !(str[i - 1] >='0' && str[i - 1] <= '9' || str[i - 1] == ')'))){
                    // int j = i + 1, x = 0;
                    // while(j < str.size() && isdigit(str[j])) x = 10 * x + (str[j ++] - '0');
                    // i = j - 1;
                    // num.push(-x);
                    // continue;
                    num.push(0);
                }
            }
            while(op.size() && pr[op.top()] >= pr[c]) eval();
            op.push(c);
        }
    }

    while(op.size() && op.top() != '(') eval();

    cout << num.top() << endl;
}