[LeetCode]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)

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);}}}}

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