Stepping Numbers

• Time : $O(2^log{10}B)$
• Space : $O(2^log{10}B)$

void helper(int l, int r, int now, vector<int>& res) {
    if(l <= now and now <= r) res.push_back(now);
    if(r < now) return;
    int b = now % 10;
    if(0 <= b + 1 and b + 1 <= 9) helper(l,r,now * 10 + b + 1, res);
    if(0 <= b - 1 and b - 1 <= 9) helper(l,r,now * 10 + b - 1, res);
}
vector<int> Solution::stepnum(int A, int B) {
    vector<int> res;
    for(int i = 1; i <= 9; i++) helper(A,B,i,res);
    if(A == 0) res.push_back(0);
    sort(begin(res),end(res));
    return res;
}