[leetcode]: 501. Find Mode in Binary Search Tree

1.题目

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],

1\ 2/2

return [2].

给一棵二叉搜索树，有重复元素。找出其中重复次数最多的元素。如果有多个重复次数一样的元素，则放在一个列表中作为返回结果。

2.分析

这题考察的是BST的中序遍历，中序遍历的元素是有序的，在遍历的过程中统计元素出现的次数即可。

3.代码

中序遍历–迭代

/** * 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) {        vector<int> mode;        if (root == NULL)            return mode;        int max_c = -1;//记录历史最多出现次数        int last_v = INT_MIN;        int count = 1;//当前值出现的次数        stack<TreeNode*> nodes;        while (!nodes.empty() || root) {            if (root) {                nodes.push(root);                root = root->left;//先遍历到左子树最下边            }            else {                root = nodes.top();                nodes.pop();                if (last_v != INT_MIN) {                    if (root->val == last_v)                        ++count;                    else                        count = 1;                }                if (count > max_c)                {                    max_c = count;                    mode.erase(mode.begin(), mode.end());                    mode.push_back(root->val);                }                else if (count == max_c)                    mode.push_back(root->val);                last_v = root->val;                root = root->right;//转到右子树            }        }        return mode;    }};

中序遍历–递归

class Solution {public:    void inOrder(TreeNode* root, vector<int>&mode,int& val,int& count,int& maxc) {        if (root == NULL)            return;        inOrder(root->left,mode,val,count,maxc);        if (val != INT_MIN) {            if (val == root->val)                ++count;            else                count = 1;        }        if (count > maxc) {            maxc = count;            mode.erase(mode.begin(), mode.end());//出现次数更多的元素，清空容器            mode.push_back(root->val);        }        else if (count == maxc)            mode.push_back(root->val);        val = root->val;        inOrder(root->right, mode, val, count, maxc);    }    vector<int> findMode(TreeNode* root) {        vector<int> mode;        if (root == NULL)            return mode;        int maxc = -1;        int count = 1;        int val = INT_MIN;        inOrder(root, mode, val, count, maxc);        return mode;    }};