题解 | #数字字符串转化成IP地址#

import java.util.*;

public class Solution {

ArrayList<String> res = new ArrayList<>();
ArrayList<String> ip = new ArrayList<>();  //存放中间结果

public ArrayList<String> restoreIpAddresses(String s) {
    dfs(s, 0);
    return res;
}
 
private void dfs(String s, int start){
    if(ip.size() == 4 && start == s.length()){  //找到一个合法解
        res.add(ip.get(0) + '.' + ip.get(1) + '.' + ip.get(2) + '.' + ip.get(3));
        return;
    }
    if(s.length() - start > 3 * (4 - ip.size()))  //剪枝
        return;
    if(s.length() - start < (4 - ip.size()))  //剪枝
        return;
    int num = 0;
    for(int i = start; i < start + 3 && i < s.length(); i++){
        num = num * 10 + (s.charAt(i) - '0');
        if(num < 0 || num > 255)  //剪枝
            break;
        ip.add(s.substring(start, i + 1));
        dfs(s, i + 1);
        ip.remove(ip.size() - 1);
         
        if(num == 0)  //不允许前缀0
            break;
    }
}

}