[LintCode] Convert Expression to Polish Notation

The basic idea is using a stack to store every operator.
For parentheses, if we meet “)” we add it into stack. if we meet”(” we pop stack till we meet “)”.

class Solution {
public:
    /** * @param expression: A string array * @return: The Polish notation of this expression */
    vector<string> convertToPN(vector<string> &expression) {
        // write your code here
        vector<string> res;
        stack<string> stk;
        for (int i = expression.size()-1; i>=0; --i) {
            string cur = expression[i];
            if(isOperator(cur)){
                if(cur == ")"){
                    stk.push(cur);
                }else if(cur == "("){
                    while(!stk.empty()){
                        string tmpOp = stk.top();
                        stk.pop();
                        if (tmpOp == ")") break;
                        res.push_back(tmpOp);
                    }
                }else{
                    while(!stk.empty() && order(cur) < order(stk.top())){
                        res.push_back(stk.top());
                        stk.pop();
                    }
                    stk.push(cur);
                }
            }else res.push_back(cur);
        }
        while(!stk.empty()) {
            res.push_back(stk.top());
            stk.pop();
        }
        reverse(res.begin(), res.end());
        return res;
    }


    bool isOperator(string s){
        if(s == "+" || s == "*" || s == "/" || s == "-" || s=="(" || s==")") return true;
        else return false;
    }
    int order(string s){
        if(s == "*" || s=="/") return 2;
        else if(s=="+" || s=="-") return 1;
        else return 0;
    }

};

The code has O(n) time complexity with O(n) memory.