Restore IP Addresses(DFS的巅峰)

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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)

DFS, 没见过这么复杂的，debug走一遍

import java.util.*;public class Solution {public List<String> restoreIpAddresses(String s) {List<String> result = new ArrayList<String>();List<String> list = new ArrayList<String>();if (s.length() < 4 || s.length() > 12)return result;helper(result, list, s, 0);return result;}public void helper(List<String> result, List<String> list, String s,int start) {if (list.size() == 4) {if (start != s.length())return;StringBuffer sb = new StringBuffer();for (String tmp : list) {sb.append(tmp);sb.append(".");}sb.deleteCharAt(sb.length() - 1);result.add(sb.toString());return;}for (int i = start; i < s.length() && i <= start + 3; i++) {String tmp = s.substring(start, i + 1);if (isvalid(tmp)) {list.add(tmp);helper(result, list, s, i + 1);list.remove(list.size() - 1);}}}private boolean isvalid(String s) {if (s.charAt(0) == '0')return s.equals("0"); // to eliminate cases like "00", "10"int digit = Integer.valueOf(s);return digit >= 0 && digit <= 255;}public static void main(String[] args) {Solution solution = new Solution();String s = "25525511135";List res = solution.restoreIpAddresses(s);System.out.println(res);}}

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