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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],

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).

题目要求找出出现最多的元素多代表的mode，即可能有多个元素出现了相同最多次；

但是要求不能使用额外的内存资源，除了递归时栈调用所使用的资源。 可以使用二叉排序树的中序遍历方法，遍历找出出现次数最多的元素。

class Solution {private:    vector
   
     res;    int curval, maxcount = 0, tempcount = 0;public:    void dfs_findMode(TreeNode *root){        if (root == NULL)return;        dfs_findMode(root->left);        tempcount++;        if (root->val != curval){            curval = root->val;            tempcount = 1;        }        if (tempcount > maxcount){            maxcount = tempcount;            res.clear();            res.push_back(root->val);        }        else if (tempcount == maxcount){            res.push_back(root->val);        }        dfs_findMode(root->right);    }    vector
    
      findMode(TreeNode* root) {        if (root == NULL)return vector
     
      {};        dfs_findMode(root);        return res;    }};
     
    
   

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