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    Question

    A Java method isLeaf(TreeNode node) is intended to check

    if a given node is a leaf in a binary tree. class TreeNode {     int val;     TreeNode left, right;     TreeNode(int x) { val = x; } } public boolean isLeaf(TreeNode node) {     if (node == null) {         return false;     }     return node.left == null || node.right == null; // Potential bug here } If isLeaf is called on a node that has a left child but no right child, what will the method return?
    A true, which is correct for a leaf node. Correct Answer Incorrect Answer
    B false, which is incorrect for a leaf node. Correct Answer Incorrect Answer
    C true, which is incorrect because it's not a leaf. Correct Answer Incorrect Answer
    D false, which is correct because it's not a leaf. Correct Answer Incorrect Answer
    E A NullPointerException. Correct Answer Incorrect Answer

    Solution

    • Dry Run: o Consider a node N that has a left child (N.left is not null) but no right child (N.right is null). This is not a leaf node by definition. o Call isLeaf(N):  node == null is false.  The return statement evaluates N.left == null || N.right == null.  This becomes false || true, which evaluates to true. o So, the method returns true for a node that is not a leaf. • Why Correct Answer (C): true, which is incorrect because it's not a leaf. o The dry run confirms that the method will return true. o The explanation correctly states that this is incorrect because a node with only one child is not a leaf. A correct isLeaf implementation would use && (AND): return node.left == null && node.right == null;.

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