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);}};