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class Solution {
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  String result = null;
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  public String smallestFromLeaf(TreeNode root) {
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    build(root, new StringBuilder());
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    return result;
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  }
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  public void build(TreeNode root, StringBuilder str) {
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    if (root == null) return;
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    StringBuilder sb = new StringBuilder(str).insert(0, String.valueOf(intToChar(root.val)));
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    if (root.left == null && root.right == null) { // we are on a leaf node
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      result = result == null || sb.toString().compareTo(result) < 0 ? sb.toString() : result;
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      return;
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    }
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    build(root.left, sb); // build left child
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    build(root.right, sb); // build right child
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  }
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  // turns an int (0-25) into a Character ex: 0 -> a, 1 -> b, 2 -> c
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  public Character intToChar(int i) {
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    return (char) (i + 'a');
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  }
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}