diff --git a/src/main/java/org/nintynine/problems/BTreeP68.java b/src/main/java/org/nintynine/problems/BTreeP68.java
new file mode 100644
index 0000000..f8625a1
--- /dev/null
+++ b/src/main/java/org/nintynine/problems/BTreeP68.java
@@ -0,0 +1,161 @@
+package org.nintynine.problems;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * Problem P68: Preorder and inorder sequences of binary trees.
+ *
+ * <p>See issue #61.
+ */
+@SuppressWarnings("checkstyle:AbbreviationAsWordInName")
+public class BTreeP68 {
+
+  private BTreeP68() {
+    // utility class
+  }
+
+  /** Simple binary tree node. */
+  public static class Node {
+    char value;
+    Node left;
+    Node right;
+
+    Node(char value) {
+      this.value = value;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) {
+        return true;
+      }
+      if (!(o instanceof Node other)) {
+        return false;
+      }
+      return value == other.value
+          && Objects.equals(left, other.left)
+          && Objects.equals(right, other.right);
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(value, left, right);
+    }
+
+    @Override
+    public String toString() {
+      if (left == null && right == null) {
+        return String.valueOf(value);
+      }
+      return value
+          + "(" + (left == null ? "NIL" : left.toString())
+          + "," + (right == null ? "NIL" : right.toString()) + ")";
+    }
+  }
+
+  /** Preorder sequence for the given tree. */
+  public static List<Character> preorder(Node node) {
+    List<Character> result = new ArrayList<>();
+    preorderRec(node, result);
+    return result;
+  }
+
+  private static void preorderRec(Node node, List<Character> acc) {
+    if (node == null) {
+      return;
+    }
+    acc.add(node.value);
+    preorderRec(node.left, acc);
+    preorderRec(node.right, acc);
+  }
+
+  /** Inorder sequence for the given tree. */
+  public static List<Character> inorder(Node node) {
+    List<Character> result = new ArrayList<>();
+    inorderRec(node, result);
+    return result;
+  }
+
+  private static void inorderRec(Node node, List<Character> acc) {
+    if (node == null) {
+      return;
+    }
+    inorderRec(node.left, acc);
+    acc.add(node.value);
+    inorderRec(node.right, acc);
+  }
+
+  /**
+   * Construct a binary search tree from its preorder sequence.
+   *
+   * @param seq preorder sequence
+   * @return root of the constructed tree
+   */
+  public static Node fromPreorder(List<Character> seq) {
+    Node root = null;
+    if (seq == null) {
+      return null;
+    }
+    for (char c : seq) {
+      root = insert(root, c);
+    }
+    return root;
+  }
+
+  private static Node insert(Node node, char value) {
+    if (node == null) {
+      return new Node(value);
+    }
+    if (value < node.value) {
+      node.left = insert(node.left, value);
+    } else if (value > node.value) {
+      node.right = insert(node.right, value);
+    }
+    return node;
+  }
+
+  /**
+   * Reconstruct a tree from its preorder and inorder sequences.
+   *
+   * @param preorder preorder sequence
+   * @param inorder inorder sequence
+   * @return reconstructed tree root
+   * @throws IllegalArgumentException if inputs are invalid
+   */
+  public static Node preInTree(List<Character> preorder, List<Character> inorder) {
+    if (preorder == null || inorder == null || preorder.size() != inorder.size()) {
+      throw new IllegalArgumentException("invalid traversals");
+    }
+    Map<Character, Integer> index = new HashMap<>();
+    for (int i = 0; i < inorder.size(); i++) {
+      index.put(inorder.get(i), i);
+    }
+    Index preIdx = new Index();
+    return build(preorder, 0, inorder.size() - 1, index, preIdx);
+  }
+
+  private static Node build(
+      List<Character> preorder,
+      int inStart,
+      int inEnd,
+      Map<Character, Integer> index,
+      Index preIdx) {
+    if (inStart > inEnd) {
+      return null;
+    }
+    char rootVal = preorder.get(preIdx.pos++);
+    Node node = new Node(rootVal);
+    int inIndex = index.get(rootVal);
+    node.left = build(preorder, inStart, inIndex - 1, index, preIdx);
+    node.right = build(preorder, inIndex + 1, inEnd, index, preIdx);
+    return node;
+  }
+
+  private static class Index {
+    int pos;
+  }
+}
diff --git a/src/test/java/org/nintynine/problems/BTreeP68Test.java b/src/test/java/org/nintynine/problems/BTreeP68Test.java
new file mode 100644
index 0000000..13dc60b
--- /dev/null
+++ b/src/test/java/org/nintynine/problems/BTreeP68Test.java
@@ -0,0 +1,103 @@
+package org.nintynine.problems;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class BTreeP68Test {
+
+    private BTreeP68.Node exampleTree() {
+        BTreeP68.Node a = new BTreeP68.Node('A');
+        BTreeP68.Node b = new BTreeP68.Node('B');
+        BTreeP68.Node c = new BTreeP68.Node('C');
+        BTreeP68.Node d = new BTreeP68.Node('D');
+        BTreeP68.Node e = new BTreeP68.Node('E');
+        BTreeP68.Node f = new BTreeP68.Node('F');
+        BTreeP68.Node g = new BTreeP68.Node('G');
+        a.left = b;
+        a.right = c;
+        b.left = d;
+        b.right = e;
+        c.right = f;
+        f.left = g;
+        return a;
+    }
+
+    @Test
+    void testPreorderAndInorder() {
+        BTreeP68.Node root = exampleTree();
+        List<Character> pre = BTreeP68.preorder(root);
+        List<Character> in = BTreeP68.inorder(root);
+        assertEquals(List.of('A','B','D','E','C','F','G'), pre);
+        assertEquals(List.of('D','B','E','A','C','G','F'), in);
+    }
+
+    @Test
+    void testFromPreorder() {
+        List<Character> seq = List.of('F','C','A','E','H','G');
+        BTreeP68.Node root = BTreeP68.fromPreorder(seq);
+        assertEquals(seq, BTreeP68.preorder(root));
+        List<Character> inorder = BTreeP68.inorder(root);
+        assertEquals(List.of('A','C','E','F','G','H'), inorder); // inorder sorted
+    }
+
+    @Test
+    void testPreInTree() {
+        BTreeP68.Node root = exampleTree();
+        List<Character> pre = BTreeP68.preorder(root);
+        List<Character> in = BTreeP68.inorder(root);
+        BTreeP68.Node rebuilt = BTreeP68.preInTree(pre, in);
+        assertEquals(pre, BTreeP68.preorder(rebuilt));
+        assertEquals(in, BTreeP68.inorder(rebuilt));
+        assertEquals(root, rebuilt);
+    }
+
+    @Test
+    void testPreInTreeInvalid() {
+        assertThrows(IllegalArgumentException.class, () ->
+                BTreeP68.preInTree(List.of('A'), List.of('A','B')));
+    }
+
+    @Test
+    void testNodeEqualityAndHash() {
+        BTreeP68.Node n1 = new BTreeP68.Node('A');
+        n1.left = new BTreeP68.Node('B');
+        BTreeP68.Node n2 = new BTreeP68.Node('A');
+        n2.left = new BTreeP68.Node('B');
+        assertEquals(n1, n2); // same structure
+        assertEquals(n1, n1); // self equality
+        assertNotEquals(n1, "other");
+        assertNotEquals(n1, null);
+        assertEquals(n1.hashCode(), n2.hashCode());
+
+        assertNotEquals(n1, new BTreeP68.Node('X'));
+        BTreeP68.Node n3 = new BTreeP68.Node('A');
+        n3.left = new BTreeP68.Node('B');
+        n3.right = new BTreeP68.Node('C');
+        assertNotEquals(n1, n3);
+
+        n2.left.value = 'C';
+        assertNotEquals(n1, n2);
+    }
+
+    @Test
+    void testNodeToString() {
+        BTreeP68.Node leaf = new BTreeP68.Node('X');
+        assertEquals("X", leaf.toString());
+
+        BTreeP68.Node root = new BTreeP68.Node('A');
+        root.left = new BTreeP68.Node('B');
+        root.right = new BTreeP68.Node('C');
+        assertEquals("A(B,C)", root.toString());
+
+        BTreeP68.Node half = new BTreeP68.Node('A');
+        half.left = new BTreeP68.Node('B');
+        assertEquals("A(B,NIL)", half.toString());
+
+        BTreeP68.Node halfRight = new BTreeP68.Node('A');
+        halfRight.right = new BTreeP68.Node('B');
+        assertEquals("A(NIL,B)", halfRight.toString());
+    }
+}