A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).
Find all strobogrammatic numbers that are of length = n.
For example,
Given n = 2, return ["11","69","88","96"].
一刷
题解：recursion, 每次n减少2，并从首尾append
public class Solution {
public List<String> findStrobogrammatic(int n) {
return helper(n, n);
}
private List<String> helper(int n, int m){
List<String> res = new ArrayList<>();
if(n == 0){
res.add("");
return res;
}
if(n == 1){
res.add("0");
res.add("1");
res.add("8");
return res;
}
List<String> list = helper(n-2, m);
for(int i=0; i<list.size(); i++){
String s = list.get(i);
if(n!=m) res.add("0" + s + "0");//not the last
res.add("1" + s + "1");
res.add("6" + s + "9");
res.add("8" + s + "8");
res.add("9" + s + "6");
}
return res;
}
}
