93. Restore IP Addresses

这道题核心思想是DFS求解，一个IP地址有四段，每段有1到3个字符，



public List<String> restoreIpAddresses(String s) {
List<String> res = new ArrayList<String>();
if (s == null || s.length() < 4 || s.length() > 12) {
return res;
}


StringBuffer tmp = new StringBuffer();
dfs(0, 0, s, tmp, res);
System.out.println(res.toString());
return res;


}


public void dfs(int count, int index, String s, StringBuffer tmp, List<String> res) {
if (count == 4 && index == s.length()) {
res.add(tmp.toString().substring(0,tmp.length()-1));
return;
}
else{
for(int i=1;i<=3 && index+i<=s.length();i++){
String tmpStr=s.substring(index,index+i);


if(isValid(tmpStr)){
int bt=tmp.length();
int ed=tmp.length()+tmpStr.length();

tmp.append(tmpStr).append(".");
dfs(count+1, index+i, s, tmp, res);
tmp.delete(bt,ed+1);
}


}
}
}

public boolean isValid(String s){
if(s.charAt(0)=='0'){
return s.equals("0");

}

Integer num=Integer.parseInt(s);
return num>0 && num<=255;


}

public static void main(String[] args) {
LeetCode code=new LeetCode();
List<String> res=code.restoreIpAddresses("25525511135");
}