RestoreIpAddress.java

public class Solution {
    public List<String> restoreIpAddresses(String s) {
		List<String> ret = new ArrayList<>();
		
		StringBuffer ip = new StringBuffer();
		for(int a = 1 ; a < 4 ; ++ a)
		for(int b = 1 ; b < 4 ; ++ b)
	        for(int c = 1 ; c < 4 ; ++ c)
		for(int d = 1 ; d < 4 ; ++ d)
		{
			if(a + b + c + d == s.length() )
			{
				int n1 = Integer.parseInt(s.substring(0, a));
				int n2 = Integer.parseInt(s.substring(a, a+b));
				int n3 = Integer.parseInt(s.substring(a+b, a+b+c));
				int n4 = Integer.parseInt(s.substring(a+b+c));
				if(n1 <= 255 && n2 <= 255 && n3 <= 255 && n4 <= 255)
				{
					ip.append(n1).append('.').append(n2)
						.append('.').append(n3).append('.').append(n4);
					if(ip.length() == s.length() + 3) ret.add(ip.toString());
					ip.delete(0, ip.length());
				}
			}
		}
		return ret;
    }
}