【DFS】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)

自己没有想出来，思路借鉴：http://blog.csdn.net/u011095253/article/details/9158449
总结一点，要求出所有组合的问题，一般用dfs；如果要求总数，用dp。

public class Solution {        public boolean isValid(String s){        if(s.charAt(0) == '0') return s.equals("0");        int num = Integer.parseInt(s);        return num > 0 && num < 256;    }        public void dfs(String s1, String s2, ArrayList<String> res, int count){//分成前后两部分进行深度优先搜索        if(count == 3 && isValid(s1)){            res.add(s2 + s1);            return ;        }                for(int i=1; i<4 && i<s1.length(); i++){            String str2 = s1.substring(0, i);            if(isValid(str2)){                String str1 = s1.substring(i);                dfs(str1, s2+str2+".", res, count+1);            }        }    }        public ArrayList<String> restoreIpAddresses(String s) {        ArrayList<String> res = new ArrayList<String>();        if(s == null) return res;        int len = s.length();        if(len <4 || len >12 ) return res;        dfs(s, "", res, 0);        return res;    }}

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