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)

看到这个题目首先就想回溯的思想，所以写出来AC的java代码如下：

public class Solution {
	private List<String> ans = new ArrayList<String>();

	public List<String> restoreIpAddresses(String s) {
		search(s, "", 0);
		return ans;
	}

	public void search(String dataString, String temp, int level) {
		// 结束条件：递归了第四层，并且dataString为空
		if (level == 4) {
			if (dataString.length() == 0) {
				ans.add(temp);
			}
		} else {
			int possible = dataString.length() > 3 ? 3 : dataString.length();
			for (int i = 1; i <= possible; i++) {
				String partofTemp = dataString.substring(0, i);
				if (isValidSegment(partofTemp)) {
					// store previous temp
					String oldTemp = temp;
					if (level == 0) {
						temp = partofTemp;
					} else {
						temp += "." + partofTemp;
					}
					search(dataString.substring(i), temp, level + 1);
					temp = oldTemp;
				}
			}
		}
	}

	public boolean isValidSegment(String s) {
		if (s.charAt(0) == '0')
			return s.length() == 1;
		Integer result = Integer.parseInt(s);
		if (result > 255)
			return false;
		return true;
	}
}

后面看到一种其他思路就是利用三个for循环进行判断。