[LeetCode] Evaluate Reverse Polish Notation

class Solution {
public:
    int stringToInt(string s)
    {
        if(s.length() == 0)
            return 0;
        int p = 1;
        if(s[0] == ‘-‘) 
        {
            p = -1;
            s = s.substr(1, s.length() - 1);
        }
        int ans = s[0] - ‘0‘;
        for(int i = 1;i < s.length();i++)
            ans = ans * 10 + s[i] - ‘0‘;
        return ans * p;
    }
    
    int evalRPN(vector<string> &tokens) {
        int len = tokens.size(), ans = 0;
        int *operand = new int[len + 1], operandNum = 0;
        memset(operand, 0, (len + 1) * sizeof(int));
        
        int res = 0; // 0 for num, 1 +, 2 -, 3 *, 4 /
        for(int i = 0;i < len;i++)
        {
            if(tokens[i] == "+")
            {
                operand[operandNum - 2] = operand[operandNum - 2] + operand[operandNum - 1];
                operandNum--;
            }
            else if(tokens[i] == "-")
            {
                operand[operandNum - 2] = operand[operandNum - 2] - operand[operandNum - 1];
                operandNum--;
            }
            else if(tokens[i] == "*")
            {
                operand[operandNum - 2] = operand[operandNum - 2] * operand[operandNum - 1];
                operandNum--;
            }
            else if(tokens[i] == "/")
            {
                operand[operandNum - 2] = operand[operandNum - 2] / operand[operandNum - 1];
                operandNum--;
            }
            else
            {
                int op = stringToInt(tokens[i]);
                operand[operandNum++] = op;
            }
        }
        return operand[0];
    }
};