<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 solve(int pos, int remainDots, string& s, vector<string>& ans, vector<string>& candidate, unordered_set<string>& st){
	
        if(pos==s.length()){
            if(remainDots==0){
                string k = "";
                for(int i=0;i<candidate.size();i++){
                    k+=candidate[i];
                    if(i!=candidate.size()-1){
                        k+='.';
                    }
                }
                ans.push_back(k);
            }
            return;
        }
        
        if(remainDots==0) return;
        
        
        string str = "";
        for(int i=pos;i<s.length();i++){
            str+=s[i];
            if(st.find(str)==st.end()) continue;
            
            candidate.push_back(str);
            solve(i+1, remainDots-1, s, ans, candidate,st);
            candidate.pop_back();
            
        }
    }
    vector<string> restoreIpAddresses(string s) {
        unordered_set<string> st;
        vector<string> ans;
        vector<string>candidate;
        for(int i=0;i<256;i++){
            st.insert(to_string(i));
        }
        solve(0, 4, s, ans, candidate, st);
        return ans;
    }
};
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