Kth Smallest Element in a BST

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

 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int kthSmallest(TreeNode root, int k) {
        if (root != null) {
        	Stack<TreeNode> stack = new Stack<>();
        	while (!stack.isEmpty() || root != null) {
        		if (root != null) {
        			stack.push(root);
        			root = root.left;
        		} else {
        			root = stack.pop();
        			if (--k == 0) {
        				return root.val;
        			}
        			root = root.right;
        		}
        	}
        }
        return 0;
    }
}