150. Evaluate Reverse Polish Notation

最新推荐文章于 2018-10-12 10:38:02 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达式的算法实现。该方法适用于包含基本算术运算符（加、减、乘、除）的表达式。通过逐个处理输入的符号，算法能正确地计算出表达式的最终结果。

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

一个栈的案例：

int evalRPN(vector<string>& tokens) {
    if (tokens.size() == 0)return 0;
    stack<int> res;
    int sum = 0;
    for (int i = 0; i < tokens.size(); i++){
        if (tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "*" && tokens[i] != "/")res.push(stoi(tokens[i]));
        else{
            int temp2 = res.top(); res.pop();
            int temp1 = res.top(); res.pop();
            if (tokens[i] == "+") res.push(temp1 + temp2);
            else if (tokens[i] == "-") res.push(temp1 - temp2);
            else if (tokens[i] == "*") res.push(temp1 * temp2);
            else if (tokens[i] == "/") res.push(temp1 / temp2);
        }
    }
    return res.top();
}