<p>A <strong>valid IP address</strong> consists of exactly four integers separated by single dots. Each integer is between <code>0</code> and <code>255</code> (<strong>inclusive</strong>) and cannot have leading zeros.</p>

<ul>
	<li>For example, <code>&quot;0.1.2.201&quot;</code> and <code>&quot;192.168.1.1&quot;</code> are <strong>valid</strong> IP addresses, but <code>&quot;0.011.255.245&quot;</code>, <code>&quot;192.168.1.312&quot;</code> and <code>&quot;192.168@1.1&quot;</code> are <strong>invalid</strong> IP addresses.</li>
</ul>

<p>Given a string <code>s</code> containing only digits, return <em>all possible valid IP addresses that can be formed by inserting dots into </em><code>s</code>. You are <strong>not</strong> allowed to reorder or remove any digits in <code>s</code>. You may return the valid IP addresses in <strong>any</strong> order.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;25525511135&quot;
<strong>Output:</strong> [&quot;255.255.11.135&quot;,&quot;255.255.111.35&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;0000&quot;
<strong>Output:</strong> [&quot;0.0.0.0&quot;]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;101023&quot;
<strong>Output:</strong> [&quot;1.0.10.23&quot;,&quot;1.0.102.3&quot;,&quot;10.1.0.23&quot;,&quot;10.10.2.3&quot;,&quot;101.0.2.3&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 20</code></li>
	<li><code>s</code> consists of digits only.</li>
</ul>

class Solution {
public:
    vector<string> path;
    vector<vector<string>> res;
    int size = 0;
    vector<string> restoreIpAddresses(string s) {
        if (s.size() > 12) {
            return {};
        }
        backtrack(s, 0, 0);
        vector<string> ans;
        for (vector<string> v : res) {
            string curPath;
            for (string s1 : v) {
                curPath += s1;
                curPath += '.';
            }
            curPath.pop_back();
            ans.push_back(curPath);
        }
        return ans;
    }
    void backtrack(const string &s, int count, int start) {

        if (count == 3) {
            string curr;
            if (start < s.size()) {
                curr = s.substr(start);
            }

            if (isValid(curr)) {
                path.push_back(curr);
                res.push_back(path);
                path.pop_back();
            }
            return;
        }
        for (int i = 1; i <= 3; ++i) {
            string curr;
            if (start < s.size()) {
                curr = s.substr(start, i);
            }

            if (isValid(curr)) {
                path.push_back(curr);
                backtrack(s, count + 1, start + i);
                path.pop_back();
            }
        }
    }
    bool isValid(const string &s) {
        if (s.size() == 0) {
            return false;
        }
        if (s.size() > 1 && s[0] == '0' && s[1] == '0') {
            return false;
        }
        for (int i = 0; i < s.size(); ++i) {
            char c = s[i];
            if (c < '0' || c > '9') {
                return false;
            }

            if (s.size() > 3) {
                return false;
            }

        }
        int index = 0;
        if (s[0] == '0' && s.size() > 1) {
            return false;
        }

        if (s.substr(index).size() == 0) {
            return false;
        }

        int val = stoi(s.substr(index));
        return val <= 255 && val >= 0;
    }

};
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