LeetCode Evaluate Reverse Polish Notation

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class Solution {public:    int evalRPN(vector<string> &tokens) {        int len = tokens.size();        if (len < 1) return 0;                vector<int> stack;        for (int i=0; i<len; i++) {            string &p = tokens[i];            if (p[0] >= ‘0‘ && p[0] <= ‘9‘ || p[0] == ‘-‘ && p.size() > 1) {                stack.push_back(stoi(p));            } else {                int slen = stack.size();                stack[slen - 2] = op(p[0], stack[slen - 2], stack[slen - 1]);                stack.pop_back();            }        }        return stack[0];    }        int op(const char op, int a, int b) {        int res = 0;        switch(op) {            case ‘+‘: res = a + b; break;            case ‘-‘: res = a - b; break;            case ‘/‘: res = a / b; break;            case ‘*‘: res = a * b; break;        }        return res;    }        int stoi(string &s) {        int len = s.size();        if (len < 1) return 0;        bool neg = s[0] == ‘-‘;        int i = neg ? 1 : 0;        int res = 0;        while (i < len) {            res = res * 10 + s[i] - ‘0‘;            i++;        }        return neg ? -res : res;    }};

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