Leetcode: 93. Restore IP Addresses

Description

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

Example

Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

思路

• 回溯，每一次選擇一個合適的數，而後進入下一次回溯，而且同時記錄part，即IP的第幾段。

代碼

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        backtrack(s, 0, 0);
        return res;
    }
    
    void backtrack(string s, int start, int part){
        if(start == s.size() && part == 4){
            res.push_back(str);
            return;
        }
            
        for(int i = start; i < s.size(); ++i){
            if(part < 4 && i - start < 3 && isValidIp(s, start, i)){
                str.append(s.substr(start, i - start + 1));
                part++;
                if(part < 4) str.push_back('.');
                
                backtrack(s, i + 1, part);
                
                if(part < 4) str.pop_back();
                part--;
                for(int j = 0; j < i - start + 1; ++j)
                    str.pop_back();
                    
            }
        }
    }
    
    bool isValidIp(string s, int start, int end){
        string tmp = s.substr(start, end - start + 1);
        int ip = stoll(tmp);
        if(s[start] == '0' && start != end) return false;
        else if(ip >= 0 && ip <= 255) return true;
        
        return false;
    }
private:
    vector<string> res;
    string str;
};