題目描述(Medium)

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

題目連結

https://leetcode.com/problems/restore-ip-addresses/description/

Example 1:

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

演算法分析

dfs，每次將當前的位數字放入ip中(ip.size()==4)，注意判斷剩餘字元的情況，以及字首不可以為0。

提交程式碼：

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> result;
        vector<string> ip;
        dfs(result, ip, s, 0);
        return result;
    }

private:
    void dfs(vector<string>& result, vector<string>& ip, string& s, int start) {
        if (ip.size() == 4 && start == s.size()) {
            result.push_back(ip[0] + '.' + ip[1] + '.' + ip[2] + '.' + ip[3]);
            return;
        }
        // 如果剩餘字元超過最大需要ip
        if ((s.size() - start) > (4 - ip.size())*3) return;
        // 如果剩餘字元不足剩餘ip所需
        if ((s.size() - start) < (4 - ip.size())) return;
        
        int num = 0;
        for (int i = start; i < start + 3; ++i) {
            num = 10 * num + s[i] - '0';
            if (num > 255 || num < 0) return;
            ip.push_back(s.substr(start, i -start + 1));
            dfs(result, ip, s, i + 1);
            ip.pop_back();
            
            if (num == 0) break; // 不允許字首為0,但允許單個0
        }
        
    }
};