LeetCode 93 复原 IP 地址

class Solution {
public:
    vector<string> res;
    int pointNum;

    bool isValid(string s, int begin, int end) {
        if (begin > end) return false; //防止在最后末尾插入.

        if (s[begin] == '0' && begin != end) return false; //第一位为0且不止一位

        int num = 0;
        for (int i = begin; i <= end; i ++) {
            if (s[i] > '9' && s[i] < '0') return false;

            num = num * 10 + (s[i] - '0');

            if (num > 255) return false;
        }
    
        return true;
    }

    void dfs(int start, string s) {
        if (pointNum == 3) {
            if (isValid(s, start, s.size() - 1)) { //判断最后子串是否合法
                res.push_back(s);
            }
            
            return;
        }
        for (int i = start; i < s.size(); i ++) {
            if (isValid(s, start, i)) {
                s.insert(s.begin() + i + 1, '.');
                pointNum ++;
                dfs(i + 2, s);
                s.erase(s.begin() + i + 1);
                pointNum --;
            }
            else break;
        }
        
    }

    vector<string> restoreIpAddresses(string s) {
        dfs(0, s);

        return res;
    }
};