Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

class Solution {
public:
    int evalRPN(vector<string> &tokens) 
    {
        stack<int> st;
        int len = tokens.size();
        int i = 0;
        int tem;
        while(i<len)
        {
            if(!isdigit(tokens[i][0])&&tokens[i].length()==1)
            {
                switch(tokens[i][0])
                {
                    case '+':
                        tem = st.top();
                        st.pop();
                        tem +=st.top();
                        st.top() = tem;
                        break;
                    case '-':
                        tem = st.top();
                        st.pop();
                        tem =st.top() - tem;
                        st.top() = tem;
                        break;
                    case '*':
                        tem = st.top();
                        st.pop();
                        tem *=st.top();
                        st.top() = tem;
                        break;
                    case '/':
                        tem = st.top();
                        st.pop();
                        tem =st.top()/tem;
                        st.top() = tem;
                        break;
                    default:
                        break;
                }
            }
            else
                {
                    tem = atoi(tokens[i].c_str());
                    st.push(tem);
                }
            i++;
        }
        return st.top();
    }
};

说明:

1. 字符串string str, 转化成c格式的操作:

str.c_str();

2. 字符串变整数,包括符号了, 负数也可以

atoi(str.c_str());

3.str可以当作数组操作