<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:
     bool valid(const string& s, int start, int length) {

        //cout<< s.substr(start, length)<<endl;

        return length == 1 ||
               (s[start] != '0' &&
                (length < 3 || length <= 3 && s.substr(start, length) <= "255"));
    }
    vector<string> res;
    void Helper(string s, int st, int end, string ans, int count){
        if(st == end){
            if(count == 4){
                ans.pop_back();
                res.push_back(ans);
            }
            return;
        }

        for(int i = st; i <end; i++){
            if(valid(s, st, i-st+1)){
                ans += s.substr(st, i-st+1) + '.';
                count++;
                Helper(s, i+1, end, ans, count);
                count--;
                for(int j = 0; j < i-st+1; j++){
                    ans.pop_back();
                }
                ans.pop_back();
            }
        }
    }
    vector<string> restoreIpAddresses(string s) {
        string ans = "";
        Helper(s, 0, s.length(), ans, 0);

        return res;
    }
};
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