逆波兰表达式，借助栈 来完成

C++ 代码

class Solution {
public:
    stack<int> sta;
    void cam(string s)
    {
        int a = sta.top();
        sta.pop();
        int b = sta.top();
        sta.pop();
        if(s == "+") sta.push(a+b);
        else if(s == "-") sta.push(b-a);
        else if(s == "*") sta.push(a*b);
        else sta.push(b/a);
    }
    int evalRPN(vector<string>& tokens) {

        int i = 0;
        while(i < tokens.size())
        {
            if(tokens[i] == "+")cam("+");
            else if(tokens[i] == "-")cam("-");
            else if(tokens[i] == "*")cam("*");
            else if(tokens[i] == "/")cam("/");
            else 
            {
                stringstream ss;
                ss << tokens[i];
                int z;
                ss >> z;
                sta.push(z);
            }
            i++;
            //cout << sta.top()<<endl;
        }

        return sta.top();
    }
};