<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:
    void helper(string& s, int index, int cnt, vector<string>& ans) {
        if (index == s.length())
            return;
        if (cnt == 3) {
            int j = 0;
            string curr;
            bool con = true;
            while (j < s.length()) {
                if (s[j] == '.') {
                    long long num = stoll(curr);
                    if ((curr[0]=='0' && curr.size()>1) || num > 255 || num < 0) {
                        con = false;
                        break;
                    }
                    curr = "";
                } else {
                    curr.push_back(s[j]);
                }
                j++;
            }
            long long num = stoll(curr);
            if ((curr[0]=='0' && curr.size()>1) || num > 255 || num < 0) {
                con = false;
            }

            if (con) {
                ans.push_back(s);
            }
            return;
        }
        helper(s, index + 1, cnt, ans);
        string p(s);
        s = s.substr(0, index + 1) + "." + s.substr(index + 1);
        helper(s, index + 2, cnt + 1, ans);
        s = p;
    }

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