Leetcode 93. Restore IP Addresses

Posted 2022-11-30 zhangwj0101

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);