<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:
int stringToInt(string s){
    int ans = 0;
    for(int i=(int)s.size()-1; i>=0; i--)
        ans += (s[i]-'0')*pow(10, s.size()-1-i);
    
    return ans;
}

bool checkString(string s){
    if(s.size() < 2)
        return true;
    if(s.size() >= 4)
        return false;
    if(s[0] == '0')
        return false;
    if(stringToInt(s) > 255)
        return false;
    
    return true;
}

void helper(string s, vector<string>& ans, int n, int i, string strNow){
    
    if(n == 3){
//            cout<<"------"<<endl;
//            cout<<s<<endl;
//            cout<<"------"<<endl;
            ans.push_back(s);
        return;
    }
    
    if(i == s.size())
        return;
    
    if(!checkString(strNow))
        return;
    
//    cout<<"(s = "<<s<<") "<<"( strNow = "<<strNow<<") "<<"(i = "<<i<<") "<<"( n = "<<n<<")"<<endl;
        
        
    if(strNow.size() != 0){
        string oldS = s;
        if(n+1 == 3){
            if(checkString(s.substr(i))){
                s = s.substr(0, i) + '.' + s.substr(i);
                helper(s, ans, n+1, i+1, "");
                s = oldS;
            }
        }
        
        else{
            s = s.substr(0, i) + '.' + s.substr(i);
            helper(s, ans, n+1, i+1, "");
            s = oldS;
        }
    }
        
    helper(s, ans, n, i+1, strNow+s[i]);
}

vector<string> restoreIpAddresses(string s){
    vector<string> ans;
    helper(s, ans, 0, 0, "");
    
//    for(int i=0; i<=ans.size()-1; i++)
//        cout<<ans[i]<<endl;
    
    return ans;
}
};
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