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class Solution {
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  public String convert(String s, int numRows) {
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    if (numRows == 1) return s;
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    StringBuilder builder = new StringBuilder();
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    for (int i = 1; i <= numRows; i++) {
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      int ind = i - 1;
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      boolean up = true;
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      while (ind < s.length()) {
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        builder.append(s.charAt(ind));
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        if (i == 1) {
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          ind += 2 * (numRows - i);
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        } else if (i == numRows) {
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          ind += 2 * (i - 1);
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        } else {
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          ind += up ? 2 * (numRows - i) : 2 * (i - 1);
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          up = !up;
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        }
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      }
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    }
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    return builder.toString();
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  }
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}