原创牛客网算法—名企高频面试题143题（16)

93. 复原IP地址

class Solution {
    List res = new ArrayList<>();
    public List restoreIpAddresses(String s) {
         List segment = new ArrayList<>();
        //开始搜索
        dfs(s, 0, segment);
        return res;
    }
        private void dfs(String s, int start, List segment) {
        if (start == s.length()) {
            if (segment.size() == 4) {
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < segment.size(); i++) {
                    sb.append(segment.get(i));
                    if (i != segment.size() - 1) sb.append('.');
                }
                res.add(sb.toString());
            }
            return;
        } else {
            if (segment.size() >= 4) return;
            for (int i = start; i < s.length() && i < start + 3; i++) {
                String str = s.substring(start, i + 1);
                if (str.charAt(0) == '0' && str.length() > 1) break;
                int num = Integer.parseInt(str);
                if (num >= 0 && num <= 255) {
                    segment.add(str);
                    dfs(s, i + 1, segment);
                    segment.remove(segment.size() - 1);
                } else {
                    break;
                }
            }
        }
    }

}

468. 验证IP地址