150. 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  根据逆波兰表达式求值  操作数入栈；遇到操作符时，操作数出栈，求值，将结果入栈；当一遍后，栈顶就是表达式的值*/class Solution {public:    int evalRPN(vector<string>& tokens) {        if(tokens.empty()) return -1;        stack<int> st;        int m=0,n=0;        for(int i=0;i<tokens.size();i++)        {            if(tokens[i]=="+" && st.size()>1)            {                m=st.top();                st.pop();                n=st.top();                st.pop();                st.push(m+n);continue;            }            if(tokens[i]=="-" && st.size()>1)            {                m=st.top();                st.pop();                n=st.top();                st.pop();                st.push(n-m);continue;            }            if(tokens[i]=="*" && st.size()>1)            {                m=st.top();                st.pop();                n=st.top();                st.pop();                st.push(m*n);continue;            }            if(tokens[i]=="/" && st.size()>1)            {                m=st.top();                st.pop();                n=st.top();                st.pop();                st.push(n/m);continue;            }            st.push(atoi(tokens[i].c_str()));        }        return st.top();    }};