Leetcode 93. Restore IP Addresses

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Question

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)

Code

public List<String> restoreIpAddresses(String s) {        if (s == null) {            return null;        }        List<String> ret = new ArrayList<>();        List<String> path = new ArrayList<>();        dfs(s, 0, path, ret);        return ret;    }    public void dfs(String s, int index, List<String> path, List<String> ret) {        if (path.size() == 4) {            if (index == s.length()) {                StringBuilder sb = new StringBuilder();                for (String str : path) {                    sb.append(str);                    sb.append('.');                }                sb.deleteCharAt(sb.length() - 1);                ret.add(sb.toString());            }            return;        }        int len = s.length();        for (int i = index; i < index + 3 && i < len; i++) {            if (s.charAt(index) == '0' && i > index) {                break;            }            String pre = s.substring(index, i + 1);            int num = Integer.parseInt(pre);            if (num > 255) {                continue;            }            path.add(pre);            dfs(s, i + 1, path, ret);            path.remove(path.size() - 1);        }    }

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