LeetCode 题解（157）: Restore IP Addresses

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 copy

class 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)