<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 {
    bool isValid(string str) {
        if (str[0] == '0') {
            return false;
        }
        int number = stoi(str);
        if (number > 255) {
            return false;
        }
        return true;
    }

    void solve(int index, string s, int size, int parts, vector<string>& ans,
               string current) {
        if (index == size && parts == 4) {
            ans.push_back(current.substr(
                0, current.size() - 1)); // consuming whole current string
            return;
        }
        if (parts > 4) {
            return;
        }

        // take one
        if (index + 1 <= size) {
            solve(index + 1, s, size, parts + 1, ans,
                  current + s.substr(index, 1) + ".");
        }

        // take 2
        if (index + 2 <= size && isValid(s.substr(index, 2))) {
            solve(index + 2, s, size, parts + 1, ans,
                  current + s.substr(index, 2) + ".");
        }
        // take 3
        if (index + 3 <= size && isValid(s.substr(index, 3))) {
            solve(index + 3, s, size, parts + 1, ans,
                  current + s.substr(index, 3) + ".");
        }
    }

public:
    vector<string> restoreIpAddresses(string s) {
        int index = 0;
        int size = s.length();
        int parts = 0; // it should be only 4 parts
        vector<string> ans;
        string current = "";

        if (size > 12) {
            return ans; // empty vector as it is not possible
        }

        solve(index, s, size, parts, ans, current);
        return ans;
    }
};
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