leetcode 150 —— 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

思路：栈

class Solution {
public:
int evalRPN(vector<string>& tokens) {
stack<int> stk;
for (int i = 0; i < tokens.size(); i++){
if (tokens[i].size()==1&&!isdigit(tokens[i][0])){
int second = stk.top();
stk.pop();
int first = stk.top();
stk.pop();
int res;
switch (tokens[i][0]){
case '+': res = first + second; break;
case '-': res = first - second; break;
case '*': res = first * second; break;
case '/': res = first / second; break;
default:
break;
}
stk.push(res);
}
else{
int tmp = s2i(tokens[i]);
stk.push(tmp);
}
}
return stk.top();
}
int s2i(string& s){
stringstream ss;
int n;
ss << s;
ss >> n;
return n;
}

};