给定一个只包含数字的字符串，复原它并返回所有可能的 IP 地址格式。
示例:
代码语言：javascript复制输入: "25525511135"
输出: ["255.255.11.135", "255.255.111.35"]解：
代码语言：javascript复制class Solution {
public List<String> restoreIpAddresses(String s) {
List<String> list = new ArrayList<>();
if (s.length() == 0) {
return list;
}
restoreIpAddresses(s, 0, "", list, 0);
return list;
}
private void restoreIpAddresses(String s, int start, String curr, List<String> list, int count) {
if (count > 4) {
return;
}
if (s.length() == start && count == 4) {
list.add(curr.substring(0, curr.length() - 1));
return;
}
// 1-digit from start
if (start 1 <= s.length()) {
restoreIpAddresses(s, start 1, curr s.substring(start, start 1) ".", list, count 1);
}
// 2-digits from start
if (start 2 <= s.length() && !s.substring(start, start 2).startsWith("0")) {
restoreIpAddresses(s, start 2, curr s.substring(start, start 2) ".", list, count 1);
}
// 3-digits from start
if (start 3 <= s.length() && !s.substring(start, start 3).startsWith("0") && Integer.parseInt(s.substring(start, start 3)) <= 255) {
restoreIpAddresses(s, start 3, curr s.substring(start, start 3) ".", list, count 1);
}
}
}
