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)
Analysis: DFS. We can optimize a little bit during the recursion to avoid compute some unnecessary cases. Note something like "10.020.255.255" is meaningless.
public class Solution { public void restoreIpAddress(String s, ArrayList<String> res, StringBuilder tem, int len, int level) { if(level == 4) { if(len==s.length()) res.add(tem.toString()); return; } for(int i=len+1; i<=len+3; i++) { if(i<=s.length() && s.length()-i>=3-level && s.length()-i<=(3-level)*3) { // optimaization String current = s.substring(len, i); if(Integer.valueOf(current)>=0 && Integer.valueOf(current)<=255) { if(current.length()>1 && current.charAt(0)=='0') continue; // cannot start from a '0' tem.append(current); if(level!=3) tem.append("."); restoreIpAddress(s, res, tem, i, level+1); if(level!=3) tem.delete(tem.length()-1-(i-len), tem.length()); else tem.delete(tem.length()-(i-len), tem.length()); } } } return; } public ArrayList<String> restoreIpAddresses(String s) { ArrayList<String> res = new ArrayList<String>(); StringBuilder tem = new StringBuilder(); restoreIpAddress(s, res, tem, 0, 0); // the init value of level should be 0!!! return res; }}
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