How to Fix Kth Smallest Element in BST Errors

In this tutorial, you'll learn about How to Fix Kth Smallest Element in BST Errors. We cover key concepts, practical examples, and best practices.

Fix kth smallest element in bst errors when inorder doesn't early-terminate or k tracking fails across calls.

Quick Fix

Wrong

def kth(r,k):
    vals=[]
    def dfs(n):
        if not n or len(vals)>=k: return
        dfs(n.left); vals.append(n.val); dfs(n.right)
    dfs(r)
    return vals[-1]

O(n) space for vals. Can't early-exit cleanly.

Right

def kth(r,k):
    st=[]; cur=r
    while st or cur:
        while cur: st.append(cur); cur=cur.left
        cur=st.pop(); k-=1
        if k==0: return cur.val
        cur=cur.right
    return -1

BST [3,1,4,null,2], k=1 -> 1. k=3 -> 3. O(k+log n).

Prevention

Iterative inorder. Decrement k on each visited node. Stop when k=0.

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