LeetCode 题解（157）: Restore IP Addresses

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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)

题解：

Backtracing，注意值必须有效，如“0”有效，而“01”，“001”， “010”等无效，值必须[0, 255]。

C++版：

class Solution {public:    vector<string> restoreIpAddresses(string s) {        vector<string> results;        if(s.length() < 4 || s.length() > 12)            return results;        int start = max(1, (int)s.length() - 9) - 1;        int end = min(3, (int)s.length() - 3) - 1;        for(int i = start; i <= end; i++) {            string result = s.substr(0, i + 1);            if(result.length() == 3 && atoi(result.c_str()) > 255)                break;            if(result[0] == '0' && result.length() > 1)                continue;            restoreIP(s.substr(i + 1, s.length() - i - 1), results, result, 2);        }        return results;    }        void restoreIP(string s, vector<string>& results, string result, int count) {        if(count == 0) {            if(s.length() == 3 && atoi(s.c_str()) > 255)                return;            if(s[0] == '0' && s.length() > 1)                return;            result += ".";            result += s;            results.push_back(result);            return;        }        int start = max(1, (int)s.length() - count * 3) - 1;        int end = min(3, (int)s.length() - count) - 1;        for(int i = start; i <= end; i++) {            string temp = result;            string part = s.substr(0, i + 1);            if(part.length() == 3 && atoi(part.c_str()) > 255)                break;            if(part[0] == '0' && part.length() > 1)                continue;            temp += ".";            temp += part;            restoreIP(s.substr(i + 1, s.length() - i - 1), results, temp, count - 1);        }    }};

Java版：

public class Solution {    public List<String> restoreIpAddresses(String s) {        List<String> results = new ArrayList<String>();        if(s.length() < 4 || s.length() > 12)            return results;        int start = Math.max(1, s.length() - 9) - 1;        int end = Math.min(3, s.length() - 3) - 1;        for(int i = start; i <= end; i++) {            String result = s.substring(0, i + 1);            if(result.length() == 3 && Integer.parseInt(result) > 255)                break;            if(result.charAt(0) == '0' && result.length() > 1)                continue;            StringBuffer sb = new StringBuffer(result);            restoreIP(s.substring(i + 1, s.length()), results, sb, 2);        }        return results;    }        public void restoreIP(String s, List<String> results, StringBuffer sb, int count) {        if(count == 0) {            if(s.length() == 3 && Integer.parseInt(s) > 255)                return;            if(s.charAt(0) == '0' && s.length() > 1)                return;            sb.append(".");            sb.append(s);            results.add(sb.toString());            return;        }        int start = Math.max(1, s.length() - count * 3) - 1;        int end = Math.min(3, s.length() - count) - 1;        for(int i = start; i <= end; i++) {            String part = s.substring(0, i + 1);            if(part.length() == 3 && Integer.parseInt(part) > 255)                break;            if(part.charAt(0) == '0' && part.length() > 1)                continue;            StringBuffer sb1 = new StringBuffer(sb.toString());            sb1.append(".");            sb1.append(part);            restoreIP(s.substring(i + 1, s.length()), results, sb1, count - 1);        }    }}

Python版：

import copyclass Solution:    # @param {string} s    # @return {string[]}    def restoreIpAddresses(self, s):        if len(s) < 4 or len(s) > 12:            return []                    start, end, results = max(1, len(s) - 9) - 1, min(3, len(s) - 3), []        for i in range(start, end):            result = s[0:i+1]            if len(result) == 3 and int(result) > 255:                break            if result[0] == '0' and len(result) > 1:                continue            self.restoreIP(s[i+1:], results, result, 2)        return results            def restoreIP(self, s, results, result, count):        if count == 0:            if len(s) == 3 and int(s) > 255:                return            if s[0] == '0' and len(s) > 1:                return            result += "."            result += s            results.append(result)                start, end = max(1, len(s) - count * 3) - 1, min(3, len(s) - count)        for i in range(start, end):            temp = copy.copy(result)            part = s[0:i+1]            if len(part) == 3 and int(part) > 255:                break            if part[0] == '0' and len(part) > 1:                continue            temp += "."            temp += part            self.restoreIP(s[i+1:], results, temp, count - 1)

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