[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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分析：思路比较简单，逆波兰表达式就是后缀表达式，遍历表达式，碰到数字就入栈，碰到操作符就出栈两个数字进行运算，运算结果再次入栈，直至遍历完毕。

class Solution {public:    // 判断是否是操作符    bool isOp(string s){        return (s == "*" || s == "+" || s == "-" || s == "/") ? true : false;    }    // 计算    int calc(string op, int a, int b){        if(op == "+") return a + b;        else if(op == "-") return a - b;        else if(op == "/") return a / b;        else return a * b;        return 0;    }    int evalRPN(vector<string>& tokens) {        // 逆波兰表达式是后缀表达式，先将其转换为中序表达式        if(tokens.size() == 0)            return 0;        if(tokens.size() == 1)            return atoi(tokens[0].c_str());        stack<int> s;        int right = 0;        int left = 0;        for(int i = 0; i < tokens.size(); ++i){            if(isOp(tokens[i])){                 // 碰到操作符就出栈两个数字，进行运算，并将结果入栈                right = s.top();                s.pop();                left = s.top();                s.pop();                s.push(calc(tokens[i], left, right));            }else{                s.push(atoi(tokens[i].c_str()));  // 如果是数字，转成int压入栈中            }        }        return s.top();    }};