Used tree, but got Memory Limit Exceeded, although the solution uses O(n) memory (easiest solution)

#1 
if(A==1) return "1";
    queue<string> q;
    q.push("1");
    while(!q.empty()){
        int n=q.size();
        for(int i=0;i<n;i++){
            string s=q.front();
            q.pop();
            if(stol(s)%A==0){
                return s;
            }
            q.push(s+'0');
            q.push(s+'1');
        }
    }
    return "0";.
#2

Memory grows exponentially and we have a complete binary tree structure it means if ans is of 20 digit we would store 2 pow 20 strings in queue…moreover max size of s is not 18 so long long may not work …do division through loop…