LeetCode: 150_Evaluate Reverse Polish Notation | 分析逆波兰式 | Medium

题目: Evaluate Reverse Polish Notation

Evaluatethe 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

简单题，借助一个stack就可以实现，将数值入栈，遇操作符时将两个数值出栈，计算后再入栈，如此即可实现。

int evalRPN(vector<string> &tokens)
{
    stack<int> stokens;
    for (int i = 0; i < tokens.size(); ++ i) {
        if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
            int x1 = stokens.top();
            stokens.pop();
            int x2 = stokens.top();
            stokens.pop();
            int x3 = caculate(x1, x2, tokens[i]); //计算之后再入栈
            stokens.push(x3);
        }
        else {
            stokens.push(atoi(tokens[i].c_str()));
        }
    }
    return stokens.top();
}

int caculate(int x1, int x2, string s) 
{
    int result;
    if (s == "+")
        result = x1 + x2;
    else if (s == "-") 
        result = x1 - x2;
    else if (s == "*")
        result = x1 * x2;
    else {
        if (x1 >= x2)
            result = x1 / x2;
        else
            result = x2 / x1;
    }
    return result;
}