93. Restore IP Addresses(dfs+剪枝)

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题目：

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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查找一个字符串构成的所有合法ip

代码：

public class Solution {    public List<String> restoreIpAddresses(String s) {        List<String> res = new ArrayList<String>();        List<String> ip = new ArrayList<String>();        dfs(res,ip,0,s);        return res;    }        void dfs(List<String> res,List<String> ip,int start,String s)    {    if(start==s.length()&&ip.size()==4 )    {    String temp = "";    temp=ip.get(0)+"."+ip.get(1)+"."+ip.get(2)+"."+ip.get(3);    res.add(temp);    return;    }    if(s.length()-start < (4-ip.size()))return ;    if(s.length()-start >(4-ip.size())*3)return ;    int num=0;    for(int i= start;i<start+3;i++)    {    if(i>s.length()-1)break;    num=num*10+(s.charAt(i)-'0');    if(num<0 || num>255)continue;    ip.add(""+num);    dfs(res,ip,i+1,s);    ip.remove(ip.size()-1);    if(num ==0)break;    }    }}

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