[LeetCode] 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

Solution:

需要注意operand为负数的情况。

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];

    }

};