Problem
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"]
class Solution { public ListfindStrobogrammatic(int n) { List res = new ArrayList<>(); if (n == 0) return res; if (n == 1) return Arrays.asList("0", "1", "8"); Map map = new HashMap (); map.put("0", "0"); map.put("1", "1"); map.put("6", "9"); map.put("8", "8"); map.put("9", "6"); dfs(map, new char[n], 0, n, res); return res; } private void dfs(Map map, char[] buffer, int index, int n, List res) { // when index just passed the mid point, (n+1)/2 works for both odd and even if (index == (n+1)/2) { res.add(String.valueOf(buffer)); return; } for (char ch: map.keySet()) { if (index == 0 && n > 1 && ch == "0") continue; if (index == n/2 && (ch == "6" || ch == "9")) continue; buffer[index] = ch; buffer[n-1-index] = map.get(ch); dfs(map, buffer, index+1, n, res); } } }
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