Evaluate Reverse Polish Notation

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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> stk;        for( int i = 0; i < tokens.size(); ++i){            if( tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "/" && tokens[i] != "*"){                stk.push(atoi( tokens[i].c_str()));            }            else{                int left = 0, right = 0;                if( !stk.empty()){                    right = stk.top();                    stk.pop();                }                else                    return 0;                if( !stk.empty()){                    left = stk.top();                    stk.pop();                }                if( tokens[i] == "+")                    stk.push( left + right);                else if( tokens[i] == "-")                    stk.push( left - right);                else if( tokens[i] == "/")                    stk.push( left / right);                else if( tokens[i] == "*")                    stk.push( left * right);                else                    return 0;            }        }        return stk.top();    }};

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