解題思路：
回溯

提交程式碼：回溯

class Solution {
    public List<String> restoreIpAddresses(String s) {
    	 List<String> ans=new ArrayList<String>();
    	 if(s.length()<4)	return ans;
    	 
    	 String base="";
    	 findIpAddresses(0,s,1,base,ans);
    	 return ans;
    }
    
    public
 
 void findIpAddresses(int p,String s,int segment,
    		String base,List<String> ans) {
    	if(segment==4) {
    		if(p<s.length()-3||p>=s.length())	return;
    			String tmp=s.substring(p);
    			if(tmp.length()!=1&&tmp.charAt(0)=='0')		return;
        		if(tmp.length()==3&&
 
tmp.compareTo("255")>0)	return;
        		ans.add(base+tmp);
    		}
    	
    	for(int i=1;i<=3;i++) {
    		if(p+i>s.length()) break;
    		String tmp=s.substring(p, p+i);
    		if(tmp.length()!=1&&tmp.charAt(0)=='0')	continue;
    		if(tmp.length()==3&&tmp.compareTo("255")>
 
0)	break;
    		findIpAddresses(p+i,s,segment+1,base+tmp+".",ans);
    	}
    }
}

執行結果：