leetcode-Evaluate Reverse Polish Notation

最新推荐文章于 2022-03-25 16:52:08 发布

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本文介绍了一种解决逆波兰表达式求值问题的方法，并通过一个C++类实现。该方法利用栈来处理算术运算，包括加减乘除等基本操作。文中还对比了使用stringstream和atoi进行字符串转整数的性能差异。

Question:

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:

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        int len = tokens.size();
        int stack[5000];
        int top = -1;
        int t1 = 0;
        int t2 = 0;
        for(int i = 0 ; i < len ; i++){
            if(top >= 1){
                t1 = stack[top];
                t2 = stack[top-1];
            }
            switch(tokens[i][0]){
                case '+':stack[--top]  = t1+t2;break;
                case '*':stack[--top]  = t2*t1;break;
                case '/':stack[--top]  = t2/t1;break;
                case '-':if(tokens[i].size()==1){
                   stack[--top]  = t2-t1; break;
                }
                 default: stack[++top] = atoi(tokens[i].c_str());
                 //stringstream ss;
                //         ss<<tokens[i];
                //         ss>>stack[++top];

            }
        }
        return stack[0];
    }
};