Find Mode in Binary Search Tree

题目地址：

https://leetcode.com/problems/find-mode-in-binary-search-tree/description/

题目：

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],

   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).

解题思路：

这道题可以用一个map来存每个int的count。

代码：

Map<Integer, Integer> map;
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);
}