lc 复原IP地址

链接：https://leetcode-cn.com/explore/interview/card/bytedance/242/string/1044/

代码：

#include <string>
#include <algorithm>
using namespace std;
class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        string path[5];
        int k = 0;
        int index = 0;
        vector<string> res;
        find(s, path, k, index, res);
        return res;
    }
    bool is_valid(string s, int front, int back) {
        if(back >= s.size()) return false;
        if(back - front >= 1 && s[front] == '0') return false;
        string temp = "";
        for(int i = front; i <= back; i++) {
            temp += s[i];
        }
        // cout << temp << endl;
        int value = stoi(temp);
        if(value >= 0 && value <= 255) return true;
        else return false;
    }
    void find(string s, string path[], int k, int index, vector<string> & res) {
        if(index >= s.size()) return;
        if(k == 3) {
            // debug
            string temp = "";
            for(int i = index; i <= s.size()-1; i++) {
                temp += s[i];
            }
            // cout << temp << endl;
            // if(temp == "135") cout << "++++++++++" << endl;
            
            if(s.size() - index > 3) return;
            
            if(is_valid(s, index, s.size()-1)) {
                cout << "====" << endl;
                string temp = "";
                for(int i = index; i <= s.size()-1; i++) {
                    temp += s[i];
                }
                string ss = "";
                for(int i = 0; i < 3; i++) {
                    if(i == 0) ss += path[i];
                    else {
                        ss += ".";
                        ss += path[i];
                    }
                }
                ss += ".";
                ss += temp;
                // cout << ss << endl;
                res.push_back(ss);
            }
            return;
        }
        for(int i = 0; i <= 2; i++) {
            if(is_valid(s, index, index+i)) {
                string temp = "";
                for(int j = index; j <= index+i; j++) {
                    temp += s[j];
                }
                path[k] = temp;
                // cout << "i: " << index << " " << index + i << endl;
                // cout << "k: " << k << endl;
                // cout << "path: ";
                // for(int j = 0; j <= k; j++) {
                //     cout << path[j] << " ";
                // }
                // cout << endl;
                // cout << "------" << endl;
                if(index+i+1 < s.size() && k < 3) find(s, path, k+1, index+i+1, res);
            }
            
        }
        return;
    }
    
};

思路：深搜，不符合条件回溯，记录路径，valid 时候记录方案。