方法一：
stack：(用string比较）

class Solution {
public:string push_and_compare_stack(stack<char> sk,string s){string ans;for(auto i:s){if(i=='#'){if(!sk.empty()){sk.pop();}}else{sk.push(i);}}while(!sk.empty()){ans+=sk.top();sk.pop();}return ans;}bool backspaceCompare(string S, string T) {stack<char> Ssk;stack<char> Tsk;string s=push_and_compare_stack(Ssk,S);string t=push_and_compare_stack(Tsk,T);return s==t;} 
};

方法二：
string：

class Solution {
public:string push_and_compare(string s){string ans;for(auto i:s){if(i=='#'){if(!ans.empty()){ans.pop_back();}}else{ans.push_back(i);}}return ans;}bool backspaceCompare(string S, string T) {string s=push_and_compare(S);string t=push_and_compare(T);return s==t;} 
};