【LeetCode】93. Restore IP Addresses

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

Example:

Input: "25525511135"
Output: 
["255.255.11.135", "255.255.111.35"]

将一串字符转换成正确的IP地址，这道题参考了这位博主，点我

class Solution {
public:
    void helper(string s, int num, int pos, string path, vector<string> &res){
        if(pos==s.size() && num==4){
            res.push_back(path.substr(0, path.size()-1));///////////////////
            return;
        }
        if(s.size()-pos > 3*(4-num))
            return;
        if(pos < s.size()){
            path += s.substr(pos,1)+".";
            helper(s, num+1, pos+1, path, res);
            path = path.substr(0, path.size()-2);
        }
        if(pos<s.size()-1 && s[pos]!='0'){
            path += s.substr(pos, 2)+".";
            helper(s, num+1, pos+2, path, res);
            path = path.substr(0, path.size()-3);
        }
        if(pos<s.size()-2 && s[pos]!='0' && s.substr(pos, 3)<="255"){
            path += s.substr(pos, 3)+".";
            helper(s, num+1, pos+3, path, res);
            path = path.substr(0, path.size()-4);
        }
    }
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        string path="";
        helper(s, 0, 0, path, res);
        return res;
    }
};