<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:
    def restoreIpAddresses(self, s: str) -> List[str]:
        res = []
        self.backtracking(s, 0, [], res)
        return res
    
    def backtracking(self, s, start_index, path, res):
        if len(path) == 4:
            res.append(('.').join(path))
            return
        
        for i in range(start_index,len(s)):
            if (len(path)<3 and i - start_index + 1 <= 3) or (len(path)==3 and (i+1)==len(s)):
                if self.is_valid_ip(s,start_index,i):
                    path.append(s[start_index:(i+1)])
                    self.backtracking(s,i+1,path,res)
                    path.pop()
            

    def is_valid_ip(self, s, start, end):
        if start > end:
            return False
        if end - start + 1 > 3:
            return False
        if start != end and s[start] == '0':
            return False
        for i in range(start,end+1):
            if not s[i].isdigit():
                return False
        if int(s[start:(end+1)]) > 255:
            return False
        return True
        

        
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