LeetCode

题目：

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：

递归，然后判断是否是合法的1/4部分

package recursion;

import java.util.ArrayList;
import java.util.List;

public class RestoreIPAddresses {

    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        dfs(res, "", s, 1);
        return res;
    }
    
    private void dfs(List<String> res, String t, String s, int k) {
        if (k == 4) {
            if (isValid(s))
                res.add(t + s);
        } else {
            int n = s.length();
            for (int i = 1; i <= 4; ++i) {
                if (n >= i && isValid(s.substring(0, i))) {
                    dfs(res, t + s.substring(0, i) + ".", s.substring(i, n), k + 1);
                }
            }
        }
    }
    
    private boolean isValid(String s) {
        if (s.length() > 0 && s.length() <= 3) {
            if (s.length() > 1 && s.charAt(0) == '0') return false;
            int value = Integer.parseInt(s);
            if (value <= 255) return true;
        }
        return false;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        RestoreIPAddresses r = new RestoreIPAddresses();
        for (String s : r.restoreIpAddresses("010010")) {
            System.out.println(s);
        }
    }

}