LeetCode - Restore IP Addresses

题目：

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:

Given

"25525511135"

,

return

["255.255.11.135", "255.255.111.35"]

. (Order does not matter)

思路：

递归，然后判断是否是合法的1/4部分

package recursion;

import java.util.ArrayList;
import java.util.List;

public class RestoreIPAddresses {

public List<String> restoreIpAddresses(String s) {
List<String> res = new ArrayList<String>();
dfs(res, "", s, 1);
return res;
}

private void dfs(List<String> res, String t, String s, int k) {
if (k == 4) {
if (isValid(s))
res.add(t + s);
} else {
int n = s.length();
for (int i = 1; i <= 4; ++i) {
if (n >= i && isValid(s.substring(0, i))) {
dfs(res, t + s.substring(0, i) + ".", s.substring(i, n), k + 1);
}
}
}
}

private boolean isValid(String s) {
if (s.length() > 0 && s.length() <= 3) {
if (s.length() > 1 && s.charAt(0) == '0') return false;
int value = Integer.parseInt(s);
if (value <= 255) return true;
}
return false;
}

public static void main(String[] args) {
// TODO Auto-generated method stub
RestoreIPAddresses r = new RestoreIPAddresses();
for (String s : r.restoreIpAddresses("010010")) {
System.out.println(s);
}
}

}