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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Number Pattern 51 - Try It</title>
</head>
<body>
  <h1>Alternating Row Number Triangle (Zigzag)</h1>
  <p>Edit rows and click Run.</p>
  <pre id="output"></pre>

  <script>
    const rows = 5;
    let k = 1;
    let result = "";

    for (let i = 1; i <= rows; i++) {
      const start = k;
      const end = k + i - 1;
      const row = [];

      if (i % 2 === 1) {
        for (let v = start; v <= end; v++) row.push(v);
      } else {
        for (let v = end; v >= start; v--) row.push(v);
      }

      result += row.join(" ") + "\n";
      k = end + 1;
    }

    document.getElementById("output").textContent = result;
  </script>
</body>
</html>

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JavaScript Number Pattern 51 (Try It)