<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> restoreIpAddresses(string s) {
        vector<string> res;
        vector<string> tempRes;
        helper(s, res, tempRes, 0);
        return res;
    }

    void helper(string& s, vector<string>& res, vector<string> tempRes, int index) {
        if (tempRes.size() > 4) {
            return;
        }

        if (index == s.length() && tempRes.size() == 4) {
            string temp;
            for (int i = 0; i < 3; i++) {
                temp += tempRes[i] + ".";
            }
            temp += tempRes[3];
            res.push_back(temp);
            return;
        }

        for (int len = 1; len + index <= s.length(); len++) {
            string str = s.substr(index, len);
            if (isValid(str)) {
                tempRes.push_back(str);
                helper(s, res, tempRes, index + len);
                tempRes.pop_back();
            }
        }
    }

    bool isValid(string s) {
        int m = s.length();
        if (m <= 0 || m > 3) {
            return false;
        }

        int num = stoi(s);
        if (s[0] == '0' && m > 1) {
            return false;
        }

        if (num > 255) {
            return false;
        }

        return true;
    }
};
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