問題描述如下：

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

cpp程式碼如下：

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> nums;
        int size_=tokens.size();
        int n1=0,n2=0;
        for(int i=0;i<size_;++i){
            if(tokens[i].length()==1){
                switch(tokens[i][0]){
                    case '+':
                    case '-':
                    case '*':
                    case '/':
                        n1=nums.top();
                        nums.pop();
                        n2=nums.top();
                        nums.pop();
                        switch(tokens[i][0]){
                            case '+':
                            nums.push(n2+n1);break;
                            case '-':
                            nums.push(n2-n1);break;
                            case '*':
                            nums.push(n2*n1);break;
                            case '/':
                            nums.push(n2/n1);break;
                        }
                        continue;
                }
            }
            int n=0;
            bool neg=false;
            unsigned int j=0;
            if(tokens[i][0]=='-'){
                neg=true;
                j=1;
            }
            for(;j<tokens[i].length();++j){
                n*=10;
                n+=tokens[i][j]-'0';
            }
            if(neg)n*=-1;
             nums.push(n);
        }
        return nums.top();
    }
};