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

 

 

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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;

    }

};

数据结构栈基础，一次过

时隔一年智商又下降了：

class Solution {

public:

    int evalRPN(vector<string>& tokens) {

        stack<int> nums;

        for (string& token : tokens) {

            if (!isop(token)) {

                int d = 0;

                sscanf(token.c_str(), "%d", &d);

                nums.push(d);

            } else {

                int b = nums.top();

                nums.pop();

                int a = nums.top();

                nums.pop();

                nums.push(calc(a, b, token[0]));

            }

        }

        return nums.top();

    }

    bool isop(string& str) {

        return str.size() == 1 && (str[0] == '+' || str[0] == '-' || str[0] == '*' || str[0] == '/');

    }

    

    int calc(int a, int b, char op) {

        switch (op) {

            case '+': return a + b; break;

            case '-': return a - b; break;

            case '/': return a / b; break;

            case '*': return a * b; break;

        }

        return 0;

    }

};