Restore IP Addresses---medium

来源：互联网发布：影视制作软件下载编辑：程序博客网时间：2024/06/08 16:18

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)

将字串划分选出合适的ip地址（回溯），此记两种做法

1）迭代，三层循环嵌套，每层取出1-3位的子字串，在第三层中将符合条件的字串加入数组

2）递归，利用深度优先搜索，逐步取出1-3个子字串并递归剩余的字串

迭代方法

package soc;//Restore IP Addressesimport java.io.*;import java.util.ArrayList;import java.util.Iterator;import java.util.List;public class RIA {public static List<String> restoreIpAddresses(String s) {List<String> iplist = new ArrayList<String>();if(s.length()<4)return iplist;  //排除过短字串String ip,ip1,ip2,ip3,ip4;//使用先取后拼合成地址的原则for(int i=1;i<4;i++){ip1 = s.substring(0, i);if(!isvalid(ip1))break;for(int j=1;(j<4&&j<s.length()-i);j++){    //循环的判断条件中加入j<s.length()-i，防止截取子字串时越界ip2 = s.substring(i,i+j);if(!isvalid(ip2))break;for(int k=1;(k<4&&k<s.length()-i-j);k++){ip3 = s.substring(i+j,i+j+k);ip4 = s.substring(i+j+k,s.length());if((!isvalid(ip3)) || (!isvalid(ip4)))continue;else {ip="";ip+=(ip1+"."+ip2+"."+ip3+"."+ip4);}iplist.add(ip);}}}return iplist;            }public static boolean isvalid(String s){if(s.length()>3)return false;if(s.length()>1 && s.startsWith("0")) return false;if(Integer.parseInt(s)<256) return true;else return false;}public static void main(String[] args) throws IOException{BufferedReader read = new BufferedReader(new InputStreamReader(System.in));String ip = read.readLine();List<String> iplist = restoreIpAddresses(ip);Iterator<String> ite = iplist.iterator();for(;ite.hasNext();){System.out.println(ite.next());}}}

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