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)

深搜+回溯+剪枝

public class Solution {
	List<String> res = new ArrayList<String>(); 
	List<Integer> list = new ArrayList<>(); 
	public List<String> restoreIpAddresses(String s) {
		if(s.length()<4||s.length()>12) return res;
		help(s,list,0);
		return res;
	}
	
	private void help(String s,List<Integer> list,int start){
		if((s.length()-start)>(4-list.size())*3) return;
		if(list.size()==4){
			res.add(list.get(0)+"."+list.get(1)+"."+list.get(2)+"."+list.get(3));
		}
		for(int j=1;j<=3&&start+j<=s.length();j++){
			String str;
			if(list.size()==3){
				str = s.substring(start);
			}else{
				str = s.substring(start,start+j);
			}
			int it = -1;
			if(!(str.length()>=2&&str.charAt(0)=='0')) it = Integer.parseInt(str);
			if(it>=0&&it<=255){
				list.add(it);
				help(s,list,start+j);
				list.remove(list.size()-1);
			}
		}
	}
}

[LeetCode]Restore IP Addresses