题目：

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代码(首刷看解析 2024年2月3日）：

class Solution {
public:
    vector<string> res;
    bool Enligible(string& s, int left, int right) {
        if (left > right) return false;
        if (s[left] == '0' && left != right) return false;
        int num = 0;
        for (int i = left; i <= right; ++i) {
            if (s[i] - '0' > 9 || s[i] - '0' < 0) return false;
            
            num = num * 10 + (s[i] - '0');
            if (num > 255) return false;
        }
        return true;
    }
    void backtracking(string& s, int startIndex, int pointNubmber) {
        if (pointNubmber == 3) {
            if (Enligible(s, startIndex, s.size() - 1)) res.push_back(s);
            return;
        }

        for (int i = startIndex; i < s.size(); ++i) {
            if (Enligible(s, startIndex, i)) {
                s.insert(s.begin() + i + 1, '.');
                pointNubmber++;
                backtracking(s, i + 2, pointNubmber);
                pointNubmber--;
                s.erase(s.begin() + i + 1);
            } else {
                break;
            }
        }
        return;
    }
    vector<string> restoreIpAddresses(string s) {
        if (s.size() < 4) return res;
        backtracking(s, 0, 0);
        return res; 
    }
};

        哎，自己写的代码错误一堆