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Kth Smallest Element in a BST

ID: 230; medium

Solution 1 (Java)

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public int kthSmallest(TreeNode root, int k) {
Deque<TreeNode> stack = new ArrayDeque<>();
while (root != null) {
stack.push(root);
root = root.left;
}
for (int i = 0; i < k - 1; i++) {
TreeNode curr = stack.peek();
if (curr.right == null) {
curr = stack.pop();
while (!stack.isEmpty() && stack.peek().right == curr) {
curr = stack.pop();
}
} else {
curr = curr.right;
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
}
}
return stack.peek().val;
}
}

Notes