Kth Smallest Element In Tree

Given a binary search tree, write a function to find the kth smallest element in the tree.

Input : 
  2
 / \
1   3

and k = 2

Return : 2

As 2 is the second smallest element in the tree.

Method:

In order iterator

Solution:

Time: O(k)
Space: O(k)

/**
 * Definition for binary tree
 * class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) {
 *      val = x;
 *      left=null;
 *      right=null;
 *     }
 * }
 */
public class Solution {
    public int kthsmallest(TreeNode A, int B) {
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode curr = A;
        int i = 0;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            if (!stack.isEmpty()) {
                curr = stack.pop();
                i ++;
                if (i == B) {
                    return curr.val;
                }
                curr = curr.right;
            }
        }
        return -1;
    }
}