501. Find Mode in Binary Search Tree（二叉搜索树）

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最新推荐文章于 2024-07-26 23:39:38 发布

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分类专栏： leetcode 算法编程文章标签： leetcode 数据结构二叉搜索树

本文链接：https://blog.youkuaiyun.com/weixin_42130471/article/details/80402680

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本文介绍了一种算法，用于在含有重复元素的二叉搜索树中寻找出现频率最高的元素（众数）。通过后序遍历的方式统计每个节点出现的次数，并返回出现次数最多的节点值。文章提供了一个具体的示例，演示了如何使用该算法解决问题。

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题目描述：

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.

For example:
Given BST [1,null,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).

代码：

class Solution {
public:
    vector<int> findMode(TreeNode* root) {
        vector<int>res;
        unordered_map<int,int>m;
        int mx=0;
        postorder(root,m,mx);
        for(auto a:m){
            if(a.second==mx)
                res.push_back(a.first);
        }
        return res;
    }
    TreeNode *postorder(TreeNode *root,unordered_map<int,int>&m,int&mx){
        if(!root)return NULL;
        postorder(root->left,m,mx);
        postorder(root->right,m,mx);
        mx=max(mx,++m[root->val]);
        return root;
    }
};