Clean and Simple Java Solution :) - Just use additional TreeNode to store needed values.

  • 0

    Here's my implementation.

    public int kthSmallest(TreeNode root, int k) {
        TreeNode result = new TreeNode(k); // using tree.val to store k, and tree.left to point to kth element
        recurse(root, result);
        return result.left.val;
    public void recurse(TreeNode root, TreeNode result) {
        if(root == null || result.left != null) return;
        recurse(root.left, result);
        result.val = result.val - 1;
        if(result.val == 0) {
            result.left = root; // kth element
        recurse(root.right, result);

  • -1

    Logic is to use a TreeNode itself to store needed values. Use a TreeNode to store k and kth TreeNode as left/right node of it.

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