230.Kth Smallest Element in a BST

1-BST InOrder Traversal Using Stack

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        int cnt = 0;
        int ans = 0;
        Deque<TreeNode> stack = new ArrayDeque<>();
        while(root != null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            cnt += 1;
            if(cnt == k){
                ans = root.val;
                break;
            }
            root = root.right;
        }
        return ans;
    }
}

2-DFS Recursive

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        int count = countNodes(root.left);
        if(k <= count){
            return kthSmallest(root.left, k);
        }
        else if (k > count + 1){
            return kthSmallest(root.right, k - 1 - count);
        }
        return root.val;
    }
    public int countNodes(TreeNode root){
        if(root == null){
            return 0;
        }
        return  1 + countNodes(root.left) + countNodes(root.right);
    }
}