<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:
    vector<string> ans;
    vector<string> IpAddress;
    vector<string> restoreIpAddresses(string s) {
        if(s.length() > 12) return {};
        doRestoreIp(0, s, "");
        return ans;
    }
    void doRestoreIp(int index, string s, string curIpAddress){
        //Stop search
        if(!isValidSubIp(curIpAddress)){
            return;
        }

        //Get One answer
        if(index == s.length()){
            if(IpAddress.size()>4)
                return; 
            if(IpAddress.size() == 4 && curIpAddress.empty()){
                string validIPAddress;
                for(string ip: IpAddress){
                    validIPAddress += ip;
                    validIPAddress += '.';
                }
                validIPAddress.pop_back();
                ans.push_back(validIPAddress);
            }
        }
        else{
            //Not placeing a dot
            doRestoreIp(index+1, s, curIpAddress+s[index]);
        }

        //Place a dot here
        if(curIpAddress.length() > 0){
            IpAddress.push_back(curIpAddress);
            doRestoreIp(index, s, "");
            IpAddress.pop_back();
        }

        return;

    }
    bool isValidSubIp(string curIpAddress){
        //Has to be 3 digits or less
        if(curIpAddress.length() > 3) return false;

        if(curIpAddress.length() == 0) return true;

        //Has to be in [0,255]
        if(curIpAddress.length() > 0 && stoi(curIpAddress) > 255 || stoi(curIpAddress) < 0) return false;

        //No leading zeros
        if(curIpAddress.length()>1 && curIpAddress[0] == '0') return false;

        return true;
    }
};
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