93. Restore IP Addresses

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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)

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DFS算法

public class Solution {    boolean dfs(String input, int number, String ipAddress, ArrayList<String> result) {        if(input.length()==0) {            return false;        }        if(number==3) {           int addressNumber = Integer.parseInt(input);           if(input.charAt(0)=='0') {               if(!(input.length()==1&&addressNumber==0))                return false;           }           if(addressNumber<=255) {               ipAddress = ipAddress + input;               result.add(ipAddress);               return true;           } else {               return false;           }        } else {            if(input.length()>=1) {                dfs(input.substring(1),number + 1,ipAddress + input.substring(0,1) + "." ,result);            }                        if(input.length()>=2&&input.charAt(0)!='0') {                dfs(input.substring(2),number + 1,ipAddress + input.substring(0,2) + "." ,result);            }                        if(input.length()>=3&&input.charAt(0)!='0') {                int addressNumber = Integer.parseInt(input.substring(0,3));                if(addressNumber<=255) {                     dfs(input.substring(3),number + 1,ipAddress + input.substring(0,3) + "." ,result);                }            }        }        return true;    }        public ArrayList<String> restoreIpAddresses(String s) {        ArrayList<String> result = new ArrayList<String>();                if(s==null||s.length()>12) {            return result;        }        String temp = "";        dfs(s,0,temp,result);        return result;    }}

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