<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:
    // https://leetcode.com/problems/restore-ip-addresses/
    auto isValidNum(const string_view s) {
        if (s.length() == 0 || s.length() > 3) return false;
	    if (s.length() > 1 && s[0] == '0') return false;
        int val =0;
        for (int i=0; i < s.length(); i++) {
            val = val*10 + (s[i]-'0');
        }
        return val <= 255;
    }
    vector<vector<string_view>> ips;
    void iterate(vector<string_view>& sofar, string_view s) {
        if (sofar.size() == 4) {
             if (s.length() == 0)
                ips.push_back(sofar);
            return;
        }
        for (int i=1; i <= s.length(); i++) {
	    auto ip = s.substr(0, i);
            if (!isValidNum(ip)) {
		// cerr << "Not valid " << ip << endl;
                return;
            }
            sofar.push_back(ip);
            iterate(sofar, s.substr(i));
            sofar.pop_back();
        }
    }
    vector<string> restoreIpAddresses(string s) {
        vector<string_view> ip;
        iterate(ip, s);
        vector<string> output;
        for (auto& v : ips) {
            ostringstream os;
	    for (auto s : v) {
		if (os.tellp()) os << '.';
		os << s;
	    }
            output.push_back(os.str());
        }
        return output;
    }
};
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