501. Find Mode in Binary Search Tree

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).

题目含义：给一个二叉搜索树，求出现次数最多的数是哪个（哪些）

    private int maxMode = Integer.MIN_VALUE;
    private void readNodeValue(TreeNode root,Map<Integer,Integer> valueMap)
    {
        if (root == null) return;
        valueMap.put(root.val,valueMap.getOrDefault(root.val,0)+1);
        maxMode = Math.max(maxMode,valueMap.get(root.val));
        readNodeValue(root.left,valueMap);
        readNodeValue(root.right,valueMap);
    }
    
    
    public int[] findMode(TreeNode root) {
      Map<Integer,Integer> valueMap = new HashMap<>();
        readNodeValue(root,valueMap);
        List<Integer> temp = new ArrayList<>();
        for (Map.Entry<Integer,Integer> entry:valueMap.entrySet())
        {
            if (entry.getValue() == maxMode)
            {
                temp.add(entry.getKey());
            }
        }
        int[] modes = new int[temp.size()];
       for (int i=0;i<modes.length;i++)
       {
           modes[i] = temp.get(i);
       }
        return modes;        
    }