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"].
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| public class Solution {
private char[] validNumbers = new char[]{'0', '1', '6', '8', '9'};
private char[] singleable = new char[]{'0', '1', '8'};
public List<String> findStrobogrammatic(int n) {
assert n > 0;
List<String> result = new ArrayList<>();
if (n == 1) {
for (char c : singleable) {
result.add(String.valueOf(c));
}
return result;
}
if (n % 2 == 0) {
helper(n, new StringBuilder(), result);
} else {
helper(n - 1, new StringBuilder(), result);
List<String> tmp = new ArrayList<>();
for (String s : result) {
for (char c : singleable) {
tmp.add(new StringBuilder(s).insert(s.length() / 2, c).toString());
}
}
result = tmp;
}
return result;
}
private void helper(int n, StringBuilder sb, List<String> result) {
if (sb.length() > n) return;
if (sb.length() == n) {
if (sb.length() > 0 && sb.charAt(0) != '0') {
result.add(sb.toString());
}
return;
}
for (char c : validNumbers) {
StringBuilder tmp = new StringBuilder(sb);
String s = "" + c + findMatch(c);
tmp.insert(tmp.length() / 2, s);
helper(n, tmp, result);
}
}
private char findMatch(char c) {
switch (c) {
case '1':
return '1';
case '6':
return '9';
case '9':
return '6';
case '8':
return '8';
case '0':
return '0';
default:
return 0;
}
}
}
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