【LeetCode】150. Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

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

使用栈(Stack)，如果是操作数则压栈，如果是操作符则弹出栈顶两个元素进行相应运算之后再压栈。

最后栈中剩余元素即计算结果。

注意：操作数的顺序。

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stk;
        for(int i = 0; i < tokens.size(); i ++)
        {
            if(tokens[i] == "+")
            {
                int a = stk.top();
                stk.pop();
                int b = stk.top();
                stk.pop();
                stk.push(b+a);
            }
            else if(tokens[i] == "-")
            {
                int a = stk.top();
                stk.pop();
                int b = stk.top();
                stk.pop();
                stk.push(b-a);
            }
            else if(tokens[i] == "*")
            {
                int a = stk.top();
                stk.pop();
                int b = stk.top();
                stk.pop();
                stk.push(b*a);
            }
            else if(tokens[i] == "/")
            {
                int a = stk.top();
                stk.pop();
                int b = stk.top();
                stk.pop();
                stk.push(b/a);
            }
            else
            {
                int digit = atoi(tokens[i].c_str());
                stk.push(digit);
            }
        }
        return stk.top();
    }
};