🟡 Port problem 6 (Zigzag Conversion) from Clojure to C++23

solutions/0001-0100/0006-zigzag-conversion/README.md

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+# 6. Zigzag Conversion
+
+[LeetCode problem 6](https://leetcode.com/problems/zigzag-conversion/)
+
+- **Difficulty**: Medium
+- **Tags**: string, simulation
+
+## Description
+
+Given a string `s` and an integer `numRows`, arrange the characters of `s` in
+a zigzag pattern across `numRows` rows (down one column, then diagonally up,
+repeating), then read the rows back left to right, top to bottom.

solutions/0001-0100/0006-zigzag-conversion/solution.cpp

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+#include "solution.hpp"
+
+#include <iterator>
+#include <vector>
+
+namespace leetcode::p0006 {
+
+std::string solve(const std::string& s, int numRows) {
+    if (numRows == 1 || numRows >= std::ssize(s)) {
+        return s;
+    }
+
+    std::vector<std::string> rows(static_cast<std::size_t>(numRows));
+    int row = 0;
+    int direction = -1;
+
+    for (char c : s) {
+        rows[static_cast<std::size_t>(row)] += c;
+        if (row == 0 || row == numRows - 1) {
+            direction = -direction;
+        }
+        row += direction;
+    }
+
+    std::string result;
+    result.reserve(s.size());
+    for (const auto& r : rows) {
+        result += r;
+    }
+
+    return result;
+}
+
+} // namespace leetcode::p0006

solutions/0001-0100/0006-zigzag-conversion/solution.hpp

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+#pragma once
+
+#include <string>
+
+namespace leetcode::p0006 {
+
+// optimized: O(n) time, O(n) space — simulate the zigzag by appending each
+// character to its current row, bouncing the row index at the edges.
+std::string solve(const std::string& s, int numRows);
+
+} // namespace leetcode::p0006

solutions/0001-0100/0006-zigzag-conversion/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 6", "[p0006]") {
+    CHECK(leetcode::p0006::solve("PAYPALISHIRING", 3) == "PAHNAPLSIIGYIR");
+    CHECK(leetcode::p0006::solve("PAYPALISHIRING", 4) == "PINALSIGYAHRPI");
+    CHECK(leetcode::p0006::solve("A", 1) == "A");
+}