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501. Find Mode in Binary Search Tree 👍

  • Time: $O(n)$
  • Space: $O(\log n)$
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class Solution {
 public:
  vector<int> findMode(TreeNode* root) {
    vector<int> ans;
    int count = 0;
    int maxCount = 0;

    inorder(root, count, maxCount, ans);
    return ans;
  }

 private:
  TreeNode* pred = nullptr;

  void inorder(TreeNode* root, int& count, int& maxCount, vector<int>& ans) {
    if (root == nullptr)
      return;

    inorder(root->left, count, maxCount, ans);
    updateCount(root, count, maxCount, ans);
    inorder(root->right, count, maxCount, ans);
  }

  void updateCount(TreeNode* root, int& count, int& maxCount,
                   vector<int>& ans) {
    if (pred && pred->val == root->val)
      ++count;
    else
      count = 1;

    if (count > maxCount) {
      maxCount = count;
      ans = {root->val};
    } else if (count == maxCount) {
      ans.push_back(root->val);
    }

    pred = root;
  }
};

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class Solution {
  public int[] findMode(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    // count[0] := currCount
    // count[1] := maxCount
    int[] count = new int[2];

    inorder(root, count, ans);
    return ans.stream().mapToInt(Integer::intValue).toArray();
  }

  private TreeNode pred = null;

  private void inorder(TreeNode root, int[] count, List<Integer> ans) {
    if (root == null)
      return;

    inorder(root.left, count, ans);
    updateCount(root, count, ans);
    inorder(root.right, count, ans);
  }

  private void updateCount(TreeNode root, int[] count, List<Integer> ans) {
    if (pred != null && pred.val == root.val)
      ++count[0];
    else
      count[0] = 1;

    if (count[0] > count[1]) {
      count[1] = count[0];
      ans.clear();
      ans.add(root.val);
    } else if (count[0] == count[1]) {
      ans.add(root.val);
    }

    pred = root;
  }
}

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class Solution:
  def findMode(self, root: TreeNode | None) -> list[int]:
    self.ans = []
    self.pred = None
    self.count = 0
    self.maxCount = 0

    def updateCount(root: TreeNode | None) -> None:
      if self.pred and self.pred.val == root.val:
        self.count += 1
      else:
        self.count = 1

      if self.count > self.maxCount:
        self.maxCount = self.count
        self.ans = [root.val]
      elif self.count == self.maxCount:
        self.ans.append(root.val)

      self.pred = root

    def inorder(root: TreeNode | None) -> None:
      if not root:
        return

      inorder(root.left)
      updateCount(root)
      inorder(root.right)

    inorder(root)
    return self.ans