LeetCode 0230 - Kth Smallest Element in a BST

Kth Smallest Element in a BST

Desicription

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:

You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.

Example 1:

代码语言：javascript复制

Input: root = [3,1,4,null,2], k = 1
   3
  / 
 1   4
  
   2
Output: 1

Example 2:

代码语言：javascript复制

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / 
     3   6
    / 
   2   4
  /
 1
Output: 3

Follow up:

What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

Solution

代码语言：javascript复制

/**
 * 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 {
private:
    int count = 0;
    int result = 0;
    void dfs(TreeNode* root, int k) {
        if(root == nullptr || count > k) {
            return ;
        }
        dfs(root->left, k);
        count  ;
        if(k == count) {
            result = root->val;
            return ;
        }
        dfs(root->right, k);        
    }
public:
    int kthSmallest(TreeNode* root, int k) {
        dfs(root, k);
        return result;
    }
};

binary element function insert search

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