Find Mode in Binary Search Tree问题及解法

问题描述：

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.
示例：

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.

问题分析：
可以通过遍历统计各个值出现的次数，最终确定答案。

过程详见代码：

/**
* 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:
map<int,int> m;
int maxi;
public:
vector<int> findMode(TreeNode* root) {
vector<int> res;
maxi = 0;
travarse(m,root,maxi);
for(map<int,int>::iterator it = m.begin(); it != m.end();it++)
{
if(it->second == maxi) res.push_back(it->first);
}
return res;
}

void travarse(map<int,int>& m,TreeNode* root,int& maxi)
{
if(root == NULL) return;
if(m.count(root->val) == 0) m.insert(pair<int,int>(root->val,1));
else m[root->val]++;
if(m[root->val] > maxi) maxi = m[root->val];
travarse(m,root->left,maxi);
travarse(m,root->right,maxi);
}
};