Restore IP Addresses

Restore IP Addresses

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

一般遇到求所有可能结果的题目，首先考虑的是递归方法

class Solution {
public:

    void restore(string s, int k, string pre, vector<string>& res){
        if (s.size()<k || s.size()>3*k) {
            return;
        }
        if (k ==0){
            res.push_back(pre);
        }
        for (int i = 1; i<=3; i++){
            if (s.size() >= i && isValid(s.substr(0, i))){
                if (k==1){
                    restore(s.substr(i), k-1, pre+s.substr(0,i), res);
                }else{
                    restore(s.substr(i), k-1, pre+s.substr(0,i) + ".", res);
                }
            }
        }
    }
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        restore(s, 4, "", res);
        return res;
    }

    bool isValid(string s) {
        if (s.empty() || s.size() > 3 || (s.size() > 1 && s[0] == '0')) return false;
        int res = atoi(s.c_str());
        return res <= 255 && res >= 0;
    }
};