Question Definition
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.
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?
Java Solution
public int kthSmallest(TreeNode root, int k) {
Stack<TreeNode> stack = new Stack<>();
while(root != null){
stack.push(root);
root = root.left;
}
while(!stack.isEmpty()) {
TreeNode cur = stack.pop();
if(--k == 0) return cur.val;
if(cur.right != null){
cur = cur.right;
while(cur != null){
stack.push(cur);
cur = cur.left;
}
}
}
return -1;
}
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