题目：

　　Evaluate the value of an arithmetic expression in .

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 解题思路： 　　遇到数字则将其压入栈中，遇到运算符就弹出栈顶的两个元素做相应的运算，然后将运算结果压入栈中。 　　最后的结果保存在栈顶。 代码：

class Solution {public:    int evalRPN(vector
     
       &tokens) {        stack
      
        st;        int operand_a = 0, operand_b = 0, tmp;        stringstream ss;        vector
       
        ::iterator it;        for(it = tokens.begin(); it != tokens.end(); it++){            ss.clear();            if ((*it) == "+" ){                operand_b = st.top(); st.pop();                operand_a = st.top(); st.pop();                st.push(operand_a + operand_b);            }            else if ((*it) == "-" ){                operand_b = st.top(); st.pop();                operand_a = st.top(); st.pop();                st.push(operand_a - operand_b);            }            else if ((*it) == "*" ){                operand_b = st.top(); st.pop();                operand_a = st.top(); st.pop();                st.push(operand_a * operand_b);            }            else if ((*it) == "/" ){                operand_b = st.top(); st.pop();                operand_a = st.top(); st.pop();                st.push(operand_a / operand_b);            }            else{                ss.str(*it);                ss >> tmp;                st.push(tmp);            }        }        return st.top();    }};