$$\color{red}{算法}\color{blue}{基础课}\color{purple}{笔记and题解}\color{green}{汇总}$$

笔记：
这是栈的一道拓展题，要注意一下运算符号的优先级。
对于栈顶的运算符进行操作，然后把数值从新压入栈中即可。

代码：

#include <bits/stdc++.h>
using namespace std;
stack<int> num;
stack<char> op;
void eval() {
    int b = num.top(); num.pop();
    int a = num.top(); num.pop();
    char opt = op.top(); op.pop();
    int x;
    if (opt == '+') x = a + b;
    else if (opt == '-') x = a - b;
    else if (opt == '*') x = a * b;
    else x = a / b;
    num.push(x);
}
int main() {
    unordered_map<char, int> pr{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};
    string str; cin >> str;
    for (int i = 0; i < str.size(); i++) {
        char 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();
            op.push(c);
        }
    }
    while (op.size()) eval();
    cout << num.top() << endl;
    return 0;
}