【LeetCode】230. Kth Smallest Element in a BST

Difficulty: Medium

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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