Leetcode---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)

Have you been asked this question in an interview?

还是DFS，关键是只能分4段，所以需要在DFS的时候限制递归的层数，可以使用一个全局变量num来存储，进入递归函数时num++，从递归函数return后num--。

1. int num=0;
   
2. bool isvalid(string& s,int i,int j){
   
3.     if(i<j && s[i]=='0')
   
4.         return false;
   
5.     if(j-i+1>3)
   
6.         return false;
   
7.     else if(j-i+1==3){
   
8.         if(s[i]=='2'){
   
9.             if(s[i+1]=='5')
   
10.                 return '0'<=s[i+2] && s[i+2]<='5';
    
11.             else if('0'<=s[i+1] && s[i+1]<'5')
    
12.                 return true;
    
13.             else if(s[i+1]>'5' && s[i+1]<='9')
    
14.                 return false;
    
15.             else
    
16.                 return false;
    
17.         }
    
18.         else if(s[i]>'2' && s[i]<='9')
    
19.             return false;
    
20.         else if(s[i]=='1')
    
21.             return true;
    
22.         else
    
23.             return false;
    
24.     }
    
25.     else if(j-i+1==2)
    
26.         return '0'<=s[i] && s[i]<='9' && '0'<=s[i+1] && s[i+1]<='9';
    
27.     else
    
28.         return '0'<=s[i] && s[i]<='9';
    
29. }
    
30. vector<string> f(string& s,int i,int j){
    
31.     num++;
    
32.     vector<string> result;
    
33.     if(j-i+1>12)
    
34.         return result;
    
35.     if(num==4){
    
36.         if(isvalid(s,i,j))
    
37.             result.push_back(s.substr(i,j-i+1));
    
38.     }else{
    
39.         for(int k=i;k<j;k++){
    
40.             if(isvalid(s,i,k)){
    
41.                 vector<string> right=f(s,k+1,j);
    
42.                 for(int m=0;m<right.size();m++){
    
43.                     string tmp=s.substr(i,k-i+1)+'.'+right[m];
    
44.                     result.push_back(tmp);
    
45.                 }
    
46.                 num--;
    
47.             }
    
48.         }
    
49.     }
    
50.     return result;
    
51. }
    
52. vector<string> restoreIpAddresses(string s) {
    
53. 
    
54.     return f(s,0,s.size()-1);
    
55. }

<script>window._bd_share_config={"common":{"bdsnskey":{},"bdtext":"","bdmini":"2","bdminilist":false,"bdpic":"","bdstyle":"0","bdsize":"16"},"share":{}};with(document)0[(getelementsbytagname('head')[0]||body).appendchild(createelement('script')).src='http://bdimg.share.baidu.com/static/api/js/share.js?v=89860593.js?cdnversion='+~(-new date()/36e5)];</script>

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