Leetcode-93. Restore IP Addresses

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前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发CSDN，mcf171专栏。

博客链接：mcf171的博客

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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)

这个题目延续之前的递归大法，以及92题排错的经验。同时还可以改进的是加入一个map，但是发现字符串不长也就算了。Your runtime beats 58.55% of java submissions.

public class Solution {    public List<String> restoreIpAddresses(String s) {        List<String> results = new ArrayList<String>();        if(s.length() < 4) return results;        generateSubString(results,s,"",0,4);        return results;    }    public void generateSubString(List<String>results,String s,String preString, int k,int remains){        if(s.length() < remains) return ;        if(k > s.length() -1) return;        if(k + remains * 3 < s.length()) return;        if(remains == 1){            if(k<s.length() - 1) if(s.charAt(k) == '0') return;            int value = Integer.parseInt(s.substring(k));            StringBuffer sb = new StringBuffer(preString);            if(value <= 255){sb.append(value);results.add(sb.toString());}        }else{            if( k + remains<=s.length()){                int value =  Integer.parseInt(s.substring(k,k + 1));                if(value <= 255){StringBuffer sb = new StringBuffer(preString); sb.append(value + ".");generateSubString(results,s,sb.toString(),k+1,remains-1);}            }            if(s.charAt(k) == '0') return;            if(k + 1 + remains <= s.length()){                int value = Integer.parseInt(s.substring(k,k+2));                if(value <= 255){StringBuffer sb = new StringBuffer(preString); sb.append(value + ".");generateSubString(results,s,sb.toString(),k+2,remains-1);}            }            if(k + 2 + remains <= s.length()){                int value = Integer.parseInt(s.substring(k,k+3));                if(value <= 255){StringBuffer sb = new StringBuffer(preString); sb.append(value + ".");generateSubString(results,s,sb.toString(),k+3,remains-1);}            }        }    }}

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