Difficulty: Medium
More:【目录】LeetCode Java实现
Description
https://leetcode.com/problems/kth-smallest-element-in-a-bst/
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:
Input: root = [3,1,4,null,2], k = 1
3
/ \
1 4
\
2
Output: 1
Example 2:
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?
Intuition
Inorder traversal
Solution
public int kthSmallest(TreeNode root, int k) {
Stack<TreeNode> stk = new Stack<>();
while(root!=null || !stk.isEmpty()){
while(root!=null){
stk.push(root);
root=root.left;
}
if(--k==0)
return stk.pop().val;
root=stk.pop().right;
}
return -1;
}
Complexity
Time complexity : O(logN+k)
Space complexity : O(logN) the depth of tree
What I've learned
1. Think about inorder traversal when there's a BST.
More:【目录】LeetCode Java实现