<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>

// 2024-09-13 22:44
// sln1 23:12
class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        std::vector<string> out;
        dfs(s, 0, std::vector<std::string>(), out);
        return out;
    }

    void dfs(const std::string& s, int pos, std::vector<std::string> path,std::vector<string>& out) {
        int n = s.size();

        if (path.size() == 4) {
            if (pos == n) {
                string line = "";
                for (int i = 0; i < 4; i++) {
                    if (std::stoi(path[i])> 255) {
                        return;
                    }
                    line += path[i];
                    if (i != 3) {
                        line += ".";
                    }
                }
                out.push_back(line);
            } else {
                return;
            }
        }

        if (pos >= n) {
            return;
        }

        if (s[pos] != '0') {
            path.push_back(s.substr(pos, 1));
            dfs(s, pos+1, path, out);
            path.pop_back();

            if (pos + 1 < n) {
                path.push_back(s.substr(pos, 2));
                dfs(s, pos+2, path, out);
                path.pop_back();
            }

            if (pos + 2 < n) {
                path.push_back(s.substr(pos, 3));
                dfs(s, pos+3, path, out);
                path.pop_back();
            }

        } else {
            path.push_back(s.substr(pos, 1));
            dfs(s, pos+1, path, out);
            path.pop_back();
        }
    }
};
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