Restore IP Addresses

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)

使用dfs搜索来解决。

public class Solution {
public ArrayList<String> restoreIpAddresses(String s) {
ArrayList<String> result = new ArrayList<String>();
ArrayList<String> list = new ArrayList<String>();
if(s.length() <4 || s.length() > 12)
return result;
helper(result, list, s , 0);
return result;
}

public void helper(ArrayList<String> result, ArrayList<String> list, String s, int start){
if(list.size() == 4){
if(start != s.length())
return;
StringBuffer sb = new StringBuffer();
for(String tmp: list){
sb.append(tmp);
sb.append(".");
}
sb.deleteCharAt(sb.length()-1);
result.add(sb.toString());
return;
}
for(int i=start; i<s.length() && i<= start+3; i++){
String tmp = s.substring(start, i+1);
if(isvalid(tmp)){
list.add(tmp);
helper(result, list, s, i+1);
list.remove(list.size()-1);
}
}
}
private boolean isvalid(String s){
if(s.charAt(0) == '0')
return s.equals("0"); // to eliminate cases like "00", "10"
int digit = Integer.valueOf(s);
return digit >= 0 && digit <= 255;
}
}