11. 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

说明：使用一个数字栈即可。（也可用数组链表模拟栈）（注意负数）

inline int newPop(stack<int> &digits) {
    int v = digits.top();
    digits.pop();
    return v;
}
class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> digits;
        for(size_t id = 0; id < tokens.size(); ++id) {
            if((tokens[id][0] == '-' && tokens[id].size() > 1 ) || 
				(tokens[id][0] >= '0' && tokens[id][0] <= '9')) digits.push(atoi(tokens[id].c_str()));
            else
            {
                int v;
                switch(tokens[id][0]){
                    case '+': {
                        digits.push(newPop(digits)+newPop(digits));
                        break;
                    } 
                    case '-': {
                        v = newPop(digits);
                        digits.push(newPop(digits)-v);
                        break;
                    } 
                    case '*': {
                        digits.push(newPop(digits)*newPop(digits));
                        break;
                    } 
                    case '/': {
                        v = newPop(digits);
                        digits.push(newPop(digits)/v);
                        break;
                    } 
                }
            }
        }
        return digits.top();
    }
};