<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>

#include <format>
#include <numeric>
#include <string>
#include <vector>

class Solution {
 public:
  std::vector<std::string> restoreIpAddresses(std::string s) {
    const int n = s.size();
    std::vector<int> segments(4);
    std::vector<std::string> output;

    auto rec = [&](auto&& rec, int start, int i) -> void {
      if (i == 4) {
        if (start == n) {
          auto ip = std::accumulate(std::next(segments.begin()), segments.end(), std::to_string(segments[0]),
                                    [](std::string a, int b) { return std::format("{}.{}", a, b); });
          output.push_back(ip);
        }
      } else if (s[start] == '0') {
        segments[i] = 0;
        rec(rec, start + 1, i + 1);
      } else {
        int sum = 0;
        for (int j = start; j < n; j++) {
          sum = sum * 10 + (s[j] - '0');
          if (sum >= 0 && sum <= 255) {
            segments[i] = sum;
            rec(rec, j + 1, i + 1);
          } else {
            break;
          }
        }
      }
    };

    rec(rec, 0, 0);
    return output;
  }
};
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