题解 | #表达式求值# C++解法，搬运“四则运算”里面大佬的思路

#include <iostream>
#include <stack>

using namespace std;

int pos;

int compute(string & data)
{
    int len = data.length();
    int num = 0;
    char flag = '+';
    stack<int> st;

    while (pos < len) {
        if (data[pos] == '{' || data[pos] == '[' || data[pos] == '(') {
            pos++;
            num = compute(data);
        }

        while (pos < len && isdigit(data[pos])) {
            num = num * 10 + data[pos] -'0';
            pos++;
        }

        switch (flag) {
        case '+':
            st.push(num);
            break;
        case '-':
            st.push(-num);
            break;
        case '*':
            {
                int temp = st.top();
                temp *= num;
                st.pop();
                st.push(temp);
                break;
            }
        case '/':
            {
                int temp = st.top();
                temp /= num;
                st.pop();
                st.push(temp);
                break;
            }
        }

        num = 0;
        flag = data[pos];
        if (data[pos] == '}' || data[pos] == ']'|| data[pos] == ')') {
            pos++;
            break;
        }
        pos++;
    }

    int sum = 0;
    while (st.size()) {
        sum += st.top();
        st.pop();
    }
    return sum;
}

int main()
{
    string data;

    while (cin >> data) {
        pos = 0;
        cout << compute(data) << endl;
    }
    return 0;
}