LeetCode-EvaluteReversePolishNotation

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

分析:

求解逆波兰表达式,从公式中提取字符串,如果是操作数就push进栈中,如果是操作符就从栈中pop出两个数,用第二个数和第一个数做运算.

AC代码:

//求解逆波兰表达式
class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> num;
        for(auto token:tokens){
            if(token == "+" || token == "-" || token == "*"||token == "/"){
                int a, b, ans;
                b = num.top(); num.pop();
                a = num.top(); num.pop();
                if(token == "+")
                    ans = a + b;
                if(token == "-")
                    ans = a - b;
                if(token == "*")
                    ans = a * b;
                if(token == "/")
                    ans = a / b;
                num.push(ans);
            }
            else{
                num.push(stoi(token));
            }
        }
        return num.top();
    }
};