[leetcode]Restore IP Addresses

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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)

演算法思路：

一般而言，求出所有**，全部**，都是用窮盡法，DFS即其中一種搜尋方法，相比BFS而言，代碼簡潔，可讀性強

 1 public class Solution { 2         public List<String> restoreIpAddresses(String s) { 3         List<String> result = new ArrayList<String>(); 4         if(s == null || s.length() < 4) return result; 5         dfs(result,new StringBuilder(),s,0); 6         return result; 7     } 8      9     private void dfs(List<String> result,StringBuilder pre,String suffix,int n){10     if(n > 3 || (n == 3 && ( suffix.length() == 0 || suffix.length() > 3 || Integer.valueOf(suffix) > 255 || (suffix.length() > 1 && suffix.charAt(0) == ‘0‘))))return;11         if(n == 3 && suffix.length() <= 3){12             result.add(pre + suffix);13             return;14         }15         for(int i = 0; i < Math.min(3, suffix.length()); i++){16             StringBuilder oldPre = new StringBuilder(pre);17             String tem = suffix.substring(0, i + 1);18             int tem2Int = Integer.valueOf(tem);19             if(tem2Int > 255 || (tem.length() > 1 && tem2Int < 10)) return;20             pre = pre.append(tem).append(‘.‘);21             n++;22             dfs(result, pre, suffix.substring(i + 1), n);23             pre = oldPre;24             n--;25         }26     }27 }