将中缀式转化为逆波兰式 （栈）

class Solution {
public:
    string ReversePolishNotation(string exp) { //假设操作数都是单个数字
        map< char, int > opPriority;
        opPriority[‘+‘] = 1;
        opPriority[‘-‘] = 1;
        opPriority[‘*‘] = 2;
        opPriority[‘/‘] = 2;
        opPriority[‘(‘] = 0;
        stack rpnStack;
        stack tmpStack;
        for(int i = 0; i < exp.size(); i++){
            if(exp[i] <= ‘9‘ && exp[i] >= ‘0‘) rpnStack.push(exp[i]);
            else if(exp[i]==‘)‘){
                while(tmpStack.top()!=‘(‘){
                    rpnStack.push(tmpStack.top());
                    tmpStack.pop();
                }
                tmpStack.pop();
            }
            else if(exp[i]==‘(‘ || tmpStack.empty()) tmpStack.push(exp[i]);
            else{
                while(!tmpStack.empty() && opPriority[tmpStack.top()] > opPriority[exp[i]]) {
                    rpnStack.push(tmpStack.top());
                    tmpStack.pop();
                }
                tmpStack.push(exp[i]);
            }
        }
        while(!tmpStack.empty() ){
            rpnStack.push(tmpStack.top());
            tmpStack.pop();
        }
        string ret = "";
        while(!rpnStack.empty() ){
            ret = rpnStack.top() + ret;
            rpnStack.pop();
        }
        return ret;
    }
};