[leetcode]Restore IP Addresses

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)

算法思路：

一般而言，求出所有**，全部**，都是用穷尽法，DFS即其中一种搜索方法，相比BFS而言，代码简洁，可读性强

public class Solution {
        public List<String> restoreIpAddresses(String s) {
        List<String> result = new ArrayList<String>();
        if(s == null || s.length() < 4) return result;
        dfs(result,new StringBuilder(),s,0);
        return result;
    }
    
    private void dfs(List<String> result,StringBuilder pre,String suffix,int n){
    if(n > 3 || (n == 3 && ( suffix.length() == 0 || suffix.length() > 3 || Integer.valueOf(suffix) > 255 || (suffix.length() > 1 && suffix.charAt(0) == '0'))))return;
        if(n == 3 && suffix.length() <= 3){
            result.add(pre + suffix);
            return;
        }
        for(int i = 0; i < Math.min(3, suffix.length()); i++){
            StringBuilder oldPre = new StringBuilder(pre);
            String tem = suffix.substring(0, i + 1);
            int tem2Int = Integer.valueOf(tem);
            if(tem2Int > 255 || (tem.length() > 1 && tem2Int < 10)) return;
            pre = pre.append(tem).append('.');
            n++;
            dfs(result, pre, suffix.substring(i + 1), n);
            pre = oldPre;
            n--;
        }
    }
}