[LeetCode] Problem 501 - Find Mode in Binary Search Tree

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Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.

Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than or equal to the node’s key.
  • The right subtree of a node contains only nodes with keys greater than or equal to the node’s key.
  • Both the left and right subtrees must also be binary search trees.

For example:

Given BST [1,null,2,2],

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1
 \
  2
 /
2

return [2].

Note

If a tree has more than one mode, you can return them in any order.

Follow up

Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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private int max = 0;
private int count = 1;
private TreeNode prev = null;

public int[] findMode(TreeNode root) {
    List<Integer> modes = new ArrayList<>();
    
    inorder(root, modes);

    int[] result = new int[modes.size()];

    for (int i = 0; i < modes.size(); i++)
        result[i] = modes.get(i);

    return result;
}

private void inorder(TreeNode root, List<Integer> modes) {
    if (root == null)
        return;

    inorder(root.left, modes);

    if (prev != null) {
        if (root.val == prev.val)
            count++;
        else {
            count = 1;
        }
    }

    if (count >= max) {
        if (count > max)
            modes.clear();

        modes.add(root.val);
        max = count;
    }

    prev = root;
    inorder(root.right, modes);
}