[lintcode] - 426 Restore IP Addresses

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这道题意思大概就是给一串数字，把它所有合法的IP地址列出来，并且不care排列顺序。事实上这道题有个坑，如果你的答案的顺序与testcase的不一致照样报错。

结题思路应该很容易想到，递归回溯就行了，每次选择当前掩码的数字位数，位数不合法的忽略，合法的跳到下一段掩码继续递归即可，递归最深处保留结果。注意判断递归停止条件。

/** * @author: decaywood * @date: 2016/1/27 9:58 * * Given a string containing only digits, * restore it by returning all possible valid IP address combinations. * * Example * Given "25525511135", return * * [ *  "255.255.11.135", *  "255.255.111.35" * ] * Order does not matter. * */public class RestoreIPAddresses {    public ArrayList<String> restoreIpAddresses(String s) {        ArrayList<String> arrayList = new ArrayList<>();        doRestore(arrayList, 0, 0, s, "");        return arrayList;    }    private void doRestore(ArrayList<String> addresses, int index, int level, String str, String candidate) {        if(level > 4) return;        level++;        if(index == str.length() && level == 5) {            addresses.add(candidate.substring(0, candidate.length() - 1));            return;        }        for (int i = 1; i <= 3; i++) {            if(index + i > str.length()) continue;            String sub = str.substring(index, index + i);            if(sub.length() > 1 && sub.charAt(0) == '0') continue;            int num = Integer.parseInt(sub);            if (num <= 255) {                doRestore(addresses, index + i, level, str, candidate + num + ".");            }        }    }    public static void main(String[] args) {        new RestoreIPAddresses().restoreIpAddresses("010010");    }}

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