LeetCode [247] Strobogrammatic Number II

247. Strobogrammatic Number II

Medium

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.

Example:

Input:  n = 2
Output: ["11","69","88","96"]

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class Solution { public List<String> findStrobogrammatic(int n) { return helper(n, n); } List<String> helper(int n, int N){ List<String> list = new ArrayList<>(); if(n==0){ list.add(""); }else if(n==1){ list.addAll(Arrays.asList("0","1", "8")); }else{ List<String> tmp = helper(n-2, N); for(String s : tmp){ list.addAll(Arrays.asList("1"+s+"1", "8"+s+"8", "6"+s+"9", "9"+s+"6")); if(n<N) list.add("0"+s+"0"); } } return list; } }