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)

 

思路：典型的DFS题目，确定好下一步状态，已经结束条件。

1. ArrayList<String> res = new ArrayList<String>();
2. StringBuffer cur = new StringBuffer();
3. boolean isValid(String s1) {
4. if(s1.charAt(0)=='0') return s1.compareTo("0") == 0;  //不能用==来判断，完全相等，包含地址
5. if(Integer.parseInt(s1)>255 || Integer.parseInt(s1) < 0) return false;
6. return true;
7. }
8. void helper(String s,int start,int index) {
9. if(index == 4) { //结束条件
10. if(start == s.length()) {
11. res.add(new String(cur));
12. }
13. return;
14. }
15. for(int i=1;i<=3;i++) {
16. if(start+i-1>=s.length()) break;
17. String str = s.substring(start,start+i);
18. if(isValid(str)) {
19. int size = cur.length();
20. String tmp ="";
21. if(start!=0) {
22. tmp = "." + str;
23. } else {
24. tmp = str;
25. }
26. cur.append(tmp);
27. helper(s,start+i,index+1);
28. cur = cur.delete(size,cur.length()); //恢复前一个状态，StringBuffer有些捉急
29. }
30. }
31. }
32. public List<String> restoreIpAddresses(String s) {
33. int s1 = s.length();
34. if(s==null || s1==0) return res;
35. helper(s,0,0);
36. return res;
37. }

C++代码：

1. vector<string> res;
2. bool isValid(string &s,int start,int end) {
3. string tmp = s.substr(start,end-start+1);
4. if(tmp.at(0)=='0') return tmp=="0";
5. int k=stoi(tmp);
6. return k<=255 && k>=0;
7. }
8. void restoreIpAddressesHelper(string &s,string &cur,int count,int start) {
9. if(count==4) {
10. if(start==s.size()) {
11. res.push_back(cur);
12. }
13. return;
14. }
15. for(int i=start;i<start+3&&i<s.size();i++) {
16. if(isValid(s,start,i)) {
17. int tmp=cur.size();
18. cur+=s.substr(start,i-start+1);
19. if(count<3) cur+='.';
20. restoreIpAddressesHelper(s,cur,count+1,i+1);
21. cur.resize(tmp);
22. }
23. }
24. }
25. vector<string> restoreIpAddresses(string s) {
27. if(s.size()==0) return res;
28. string cur;
29. restoreIpAddressesHelper(s,cur,0,0);
30. return res;
31. }