Evaluate Reverse Polish Notation

标签（空格分隔）： leetcode 算法

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题目：

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

解答：

class Solution {public:    int evalRPN(vector<string>& tokens) {        stack<int> s;        for (vector<string>::size_type i = 0; i < tokens.size(); ++i) {            string& str = tokens[i];            if (str.size()) {                if (isdigit(str[0]) ||                     (str[0] == '+' && str.size() > 1) ||                     (str[0] == '-' && str.size() > 1)) {                    s.push(atoi(str.c_str()));                } else if (s.size() >= 2) {                    int rhs = s.top();                    s.pop();                    int lhs = s.top();                    s.pop();                    switch(str[0]) {                        case '+':                            s.push(lhs + rhs);                            break;                        case '-':                            s.push(lhs - rhs);                            break;                        case '*':                            s.push(lhs * rhs);                            break;                        case '/':                            if (rhs == 0) {                                return 0;                            } else {                                s.push(lhs / rhs);                                break;                            }                        default:                            return 0;                    }                } else {                    return 0;                }            }        }        if (s.size() == 1) {            int ret = s.top();            s.pop();            return ret;        } else {            return 0;        }    }};