【leetcode】【medium】93. Restore IP Addresses

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"]

题目链接：https://leetcode-cn.com/problems/restore-ip-addresses/

思路

合法ip地址：

1）一共由4个数字段组成；

2）每段数字都在0-255之间；

3）一段数字不能以0开头。

回溯法将所有可能列出，中间加判断来剪枝。

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        if(s.size()<4 || s.size()>12) return res;
        return ip(s, 1);
    }
    vector<string> ip(string s, int level){
        vector<string> res;
        if(level==4){
            if(s.size()>1 && s[0]=='0') return res;
            if(s.size()>0 && atoi(s.c_str())<=255) {
                res.push_back(s);
            }
        }else{
            for(int i = 1; i<=3; ++i){
                if(i>1 && s[0]=='0') continue;
                if(s.size()>i){
                    string head = s.substr(0, i);
                    if(atoi(head.c_str())<=255){
                        vector<string> vec = ip(s.substr(i, s.size()-i), level+1);
                        for(int j = 0; j<vec.size(); ++j){
                            vec[j] = head + "." + vec[j];
                        }
                        res.insert(res.end(), vec.begin(), vec.end());
                    }
                }
            }
        }
        return res;
    }
};