Find Unique Binary String | LeetCode | Java

class Solution { Set<String> numSet; int n = 0; String res = ""; public String findDifferentBinaryString(String[] nums) { n = nums.length; numSet = new HashSet<>(); for(String s : nums){ numSet.add(s); } backTrack("", 0); return res; } void backTrack(String str, int index){ if(str.length()==n){ if(!numSet.contains(str)){ res = str; } return; } if(index>n) return; String str0 = str + "0"; backTrack(str0, index+1); String str1 = str + "1"; backTrack(str1, index+1); return; } }