<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 {
private:
    bool check(string s) {
        int n = s.size();
        if (n > 3) {
            return false;
        }
        if (n > 1 && s[0] == '0') {
            return false;
        }
        int temp = stoi(s);
        if (temp > 255) {
            return false;
        }
        return true;
    }

public:
    vector<string> restoreIpAddresses(string s) {
        int n = s.size();
        if (n > 12)
            return {};
        vector<string> res;
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                for (int k = 1; k <= 3; k++) {
                    int sum = i + j + k;
                    if (sum < n && sum + 3 >= n) {
                        string s1 = s.substr(0, i);
                        string s2 = s.substr(i, j);
                        string s3 = s.substr(i + j, k);
                        string s4 = s.substr(sum);
                        if (check(s1) && check(s2) && check(s3) && check(s4)) {
                            res.push_back(s1 + "." + s2 + "." + s3 + "." + s4);
                        }
                    }
                }
            }
        }
        return res;
    }
};
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