LEETCODE: 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:        bool isFlag(string str) {        if(str.length() != 1)            return false;        if(str[0] == '-' || str[0] == '+' || str[0] == '/' || str[0] == '*')            return true;        return false;    }        int calculate(int left, int right, string flag) {        if(flag[0] == '+')            return left + right;        if(flag[0] == '-')            return left - right;        if(flag[0] == '*')            return left * right;        if(flag[0] == '/')            return left / right;        return 0;    }        int stringToNumber(string str) {        int number = 0;        int flag = 1;        for(int ii = 0; ii < str.length(); ii ++) {            if(ii == 0) {                if(str[0] == '-') {                    flag = -1;                    continue;                }                else if(str[0] == '+') {                    continue;                }            }            number = number * 10 + str[ii] - '0';        }        return number * flag;    }    int evalRPN(vector<string> &tokens) {        if(tokens.size() == 0)            return 0;        stack<int> st;        for(int ii = 0; ii < tokens.size(); ii ++) {            if(isFlag(tokens[ii])) {                int right = st.top();                st.pop();                int left = st.top();                st.pop();                int current  = calculate(left, right, tokens[ii]);                st.push(current);            }            else {                st.push(stringToNumber(tokens[ii]));            }        }        return st.top();    }};

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