 # [LeetCode] 247. Strobogrammatic Number II

## 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"]

## Solution

``````class Solution {
public List<String> findStrobogrammatic(int n) {
List<String> res = new ArrayList<>();
if (n == 0) return res;
if (n == 1) return Arrays.asList("0", "1", "8");
Map<Character, Character> map = new HashMap<Character, Character>();
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<Character, Character> map, char[] buffer, int index, int n, List<String> res) {
// when index just passed the mid point, (n+1)/2 works for both odd and even
if (index == (n+1)/2) {
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);
}
}
}``````