<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> restoreIpAddresses(string s) {
        vector<string> ans; string cur = "";
        auto check = [&](string s) -> bool {
            if (s.size() >= 4) return false;
            int value = 0;
            for (int i = 0; i < s.size(); i++) {
                value += pow(10, s.size() - i - 1) * (s[i] - '0');
            }
            printf("value: %d\n", value);
            if (s[0] == '0' && value != 0) return false;
            if (s[0] == '0' && value == 0 && s.size() > 1) return false;
            return 0 <= value && value <= 255;
        };
        cout << check("10") << '\n' << check("15") << '\n';
        auto append = [&](vector<string> &v) -> void {
            string res = "";
            for (int i = 0; i < v.size(); i++) {
                res += v[i];
                if (i != v.size() - 1) {
                    res += '.';
                }
            }
            ans.push_back(res);
        };
        auto dfs = [&](auto&& dfs, int ind, string s, string cur, int cnt) -> void {
            if (ind >= s.size() && cnt == 3) {
                ans.push_back(cur);
                // return ;
            }
            for (int len = 1; len <= 3; len++) {
                if (ind + len > s.size()) break;
                string block = s.substr(ind, len);
                if (!check(block)) continue;
                if (ind + len >= s.size()) {
                    dfs(dfs, ind + len, s, cur + block, cnt);
                } else {
                    dfs(dfs, ind + len, s, cur + block + '.', cnt + 1);
                }
            }
        };
        dfs(dfs, 0, s, cur, 0);
        return ans;
    }
};
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