LeetCode-501. Find Mode in Binary Search Tree (JAVA)出现次数最多的元素

501. Find Mode in Binary Search Tree

Given a binary search tree (BST) with duplicates, find all themode(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 keysless than or equal to the node's key.
• The right subtree of a node contains only nodes with keysgreater 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],

   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).

使用map记录次数，时间和空间复杂度都是O(N)，使用任何一种遍历都可

private Map<Integer, Integer> map;private int max = 0;public int[] findMode(TreeNode root) {if (root == null)return new int[0];this.map = new HashMap<>();inorder(root);List<Integer> list = new LinkedList<>();for (int key : map.keySet()) {if (map.get(key) == max)list.add(key);}int[] res = new int[list.size()];for (int i = 0; i < res.length; i++)res[i] = list.get(i);return res;}private void inorder(TreeNode node) {if (node.left != null)inorder(node.left);map.put(node.val, map.getOrDefault(node.val, 0) + 1);max = Math.max(max, map.get(node.val));if (node.right != null)inorder(node.right);}}