<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;
    string temp;
    bool check(string s) {
        if(s.size() == 0) return 0;
        int curr = 0;
        string t = "";
        if(s[0] == '.') return 0;
        for(int i = 0; i < s.size(); i++){
            if(s[i] == '.') {
                if(curr > 0 &&  t[0] == '0') return 0;
                if(curr == 0 && t.size() > 1) return 0;
                if(curr < 0 || curr > 255) return 0;
                curr = 0;
                t = "";
            } else {
                curr = curr * 10 + (int)(s[i] - '0');
                if(curr > 255) return 0;
                t.push_back(s[i]);
            }
        }
        if(curr > 0 &&  t[0] == '0') return 0;
        if(curr == 0 && t.size() > 1) return 0;
        if(curr < 0 || curr > 255) return 0;
        return 1;
    }
    void rec(int i, int cnt, string &s) {
        if(cnt >= 4) return;
        if(i == s.size()) {
            if(cnt == 3 && check(temp)) {
                ans.push_back(temp);
            }
            return;
        }
        //add .
        temp.push_back('.');
        temp.push_back(s[i]);
        rec(i+1, cnt + 1, s);
        temp.pop_back();
        temp.pop_back();
        // add element
        temp.push_back(s[i]);
        rec(i+1, cnt, s);
        temp.pop_back();
    }
    vector<string> restoreIpAddresses(string s) {
        ans.clear();
        temp.clear();
        for(auto x: s) {
            if(x < '0' || x > '9') return ans;
        }
        rec(0, 0, s);
        return ans;
    }
};
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