Day-20-KthSmallestElementinaBST.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) {
         LinkedList<TreeNode> stack = new LinkedList<TreeNode>();

        while (true) {
          while (root != null) {
            stack.add(root);
            root = root.left;
          }
          root = stack.removeLast();
          if (--k == 0) return root.val;
          root = root.right;
        }
    }
}