问题描述：

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)

基本思想：

回溯法

代码：

    //is 0  ok?
    public boolean isValidIp(String s){  
    	if(s.length() != 1 && s.startsWith("0"))
    		return false;
        int ip = Integer.valueOf(s);
        if(ip >= 0 && ip <=255)
            return true;
        return false;
    }
    public void validIp(List<String> list,String tmpIp,String allStr,int used,int i){
        int size = allStr.length();
        if(i == 5){
        	if(tmpIp.startsWith("."))
        		tmpIp = tmpIp.substring(1);
            list.add(tmpIp);
        }
        
        if(allStr.length() - used > (4-i+1)*3 ||
            allStr.length() -used < 4-i+1)
            return;
        
        int most,lest;
        if(i == 4){
            most = allStr.substring(used).length();
            lest = most;
        }
        else{    
             most = 3;
             lest = 1;
        }
        
        for(int j = lest; j<= most; j++){
        	if(j+used > size)
        		break;
            String ip = allStr.substring(used,j+used);
            boolean isIp = isValidIp(ip);
            if(isIp){
                tmpIp = tmpIp + "."+ip;
                used = used + ip.length();
                validIp(list,tmpIp,allStr,used,i+1);
                tmpIp = tmpIp.substring(0,tmpIp.length()-ip.length()-1);
                used = used-ip.length();
            }
        }
        
        
    }
    public List<String> restoreIpAddresses(String s) {
        int size = s.length();
        if(size < 4 || size >12)
            return new ArrayList<String>();
        
        List<String> result = new ArrayList<String>();
        
        String tmpIp = "";
        validIp(result,tmpIp,s,0,1);
        return result;
    }

[leetcode]Restore IP Addresses