【LeetCode】Evaluate Reverse Polish Notation

用栈。遇到数字压栈。遇到运算符则弹出前两个操作数计算结果并压栈。最后字符串矢量都处理完了就得到结果。

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        stack<int> st;
        for(int i=0;i<tokens.size();i++)
        {
            if("+"==tokens[i]||"-"==tokens[i]||"*"==tokens[i]||"/"==tokens[i])
            {
                int n1=st.top(); st.pop();
                int n2=st.top(); st.pop();
                if("+"==tokens[i]) st.push(n1+n2);
                if("-"==tokens[i]) st.push(n2-n1);
                if("*"==tokens[i]) st.push(n1*n2);
                if("/"==tokens[i]) st.push(n2/n1);
                continue;
            }
            int num=0;
            int minus=false;
            for(int j=0;j<tokens[i].length();j++)
            {
                if('-'==tokens[i][j]) {minus=true; continue;}
                num=10*num+tokens[i][j]-'0';
            }
            if(minus) num=0-num;
            st.push(num);
        }
        return st.top();
    }
};