[LeetCode]150. Evaluate Reverse Polish Notation

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

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逆波兰表达式求值，用栈解决，碰到数就入栈，碰到运算符就出栈两个数，并结合运算符计算，将结果压入栈，最后的栈顶元素就是最终的结果。

class Solution {public:    int evalRPN(vector<string>& tokens) {        stack<int> s;        for(int i = 0; i < tokens.size(); ++i){            if(tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/"){                int first = s.top();                s.pop();                int second = s.top();                s.pop();                if(tokens[i] == "+")                    s.push(second + first);                else if(tokens[i] == "-")                    s.push(second - first);                else if(tokens[i] == "*")                    s.push(second * first);                else                    s.push(second / first);            }else{                s.push(stoi(tokens[i]));                // s.push(atoi(tokens[i].c_str()));            }        }        return s.top();    }};