[LeetCode] Evaluate Reverse Polish Notation

最新推荐文章于 2021-03-20 22:13:41 发布

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本文介绍了一种使用栈的数据结构来解析并计算逆波兰表示法（Reverse Polish Notation, RPN）算术表达式的算法实现。通过遍历输入的字符串数组，遇到操作数则压入栈中，遇到运算符则从栈顶取出两个操作数进行计算，并将结果再次压入栈中，直至遍历结束，栈顶元素即为表达式的计算结果。

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

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> s;
        vector<string> ::iterator it = tokens.begin();
        int num1 = 0,num2 = 0;
        while(it != tokens.end()){
            if(*it == "+"||*it == "-" || *it == "*" || *it == "/"){
                num2 = s.top();
                s.pop();
                num1 = s.top();
                s.pop();
                if(*it == "+")
                    s.push(num1 + num2);
                if(*it == "-")
                    s.push(num1 - num2);
                if(*it == "*")
                    s.push(num1 * num2);
                if(*it == "/")
                    s.push(num1 / num2);
            }
            else
                s.push(stoi(*it));
            it ++;
        }
        return s.top();
    }
};