Java implementation LeetCode 93 rehabilitation IP address

93. Recovery IP address

Given a string contains only numeric, restoring it and returns all possible IP address format.

Example:

Input: "25525511135"
Output: [ "255.255.11.135", "255.255.111.35 "]

PS:
kneeling, was LeetCode who in the world, I learned last bit operators, the school's ip address

class Solution {
    private List<String> res = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        if (s.length() < 4) //非法输入
            return res;
        backtrack(s, 0, new StringBuilder(), 0);
        return res;
    }
    private void backtrack(String s, int start, StringBuilder sb, int pointNumOfSb) {
        if (pointNumOfSb > 4) //大于三个点，则剪枝，这里大于4是因为最后一次还会加一
            return;
        if (start == s.length() && pointNumOfSb == 4) {   //pointNumOfSb==4，则是一个合法的IP
            res.add(sb.toString().substring(1));   //substring(1)是因为每次append(".xxx")，第零个位置是"."
            return ;
        }
        for (int i = start; i < s.length() && i - start < 3; i++) { //i-start < 3，如果大于三位数则返回
            String x = s.substring(start, i + 1);
            if (x.charAt(0) == '0' && x.length() > 1) //如果是0xx这种则返回
                return ;
            if (Integer.parseInt(x) <= 255) {
                sb.append("." + x);
                backtrack(s, i + 1, sb, pointNumOfSb + 1);
                sb.delete(sb.lastIndexOf("."), sb.length());
            }
        }
    }
}