1 递归—中序遍历

【极端情况】：BST树中所有节点唯一，则所有节点均是众数

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    vector<int> vec;
    TreeNode* pre = nullptr;
    int cont = 0, maxCont = 0;
public:
    void traverse(TreeNode* cur) {
        if (!cur) return;
        traverse(cur->left);
        /**以下是中间节点操作**/
        if (pre && pre->val == cur->val) cont++;
        else cont = 0;
        // 极端情况: 所有元素均唯一，则所有元素都是众数
        if (cont == maxCont) vec.emplace_back(cur->val);
        if (cont > maxCont) {
            maxCont = cont;
            // 间接清空之前vector
            vec = vector<int> {cur->val};
        }
        pre = cur;
        /**以上是中间节点操作**/
        traverse(cur->right);
    }
    vector<int> findMode(TreeNode* root) {
        traverse(root);
        return vec;
    }
};