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],

/**
* 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 {
public:
vector<int> findMode(TreeNode* root) {
if (!root) return {};
vector<int> res;
TreeNode *p = root, *pre = NULL;
stack<TreeNode*> s;
int mx = 0, cnt = 1;;
while (!s.empty() || p) {
while (p) {
s.push(p);
p = p->left;
}
p = s.top(); s.pop();
if (pre) {
cnt = (p->val == pre->val) ? cnt + 1 : 1;
}
if (cnt >= mx) {
if (cnt > mx) res.clear();
res.push_back(p->val);
mx = cnt;
}
pre = p;
p = p->right;
}
return res;
}
};