5 1
3 1 4 null 2

1

8 3
5 3 6 2 4 null null 1

3

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def build_tree(level_order):
    if not level_order or level_order[0] == 'null':
        return None
    
    root = TreeNode(int(level_order[0]))
    queue = [root]
    i = 1
    while i < len(level_order):
        current = queue.pop(0)
        # 构造左子节点
        if i < len(level_order) and level_order[i] != 'null':
            current.left = TreeNode(int(level_order[i]))
            queue.append(current.left)
        i += 1
        # 构造右子节点
        if i < len(level_order) and level_order[i] != 'null':
            current.right = TreeNode(int(level_order[i]))
            queue.append(current.right)
        i += 1
    return root

def kthSmallest(root, k):
    # 中序遍历，计数器+返回值捕获
    count = 0
    result = None
    def inorder(node):
        nonlocal count, result
        if not node or result is not None:
            return
        inorder(node.left)
        count += 1
        if count == k:
            result = node.val
            return
        inorder(node.right)

    inorder(root)
    return result

if __name__ == "__main__":
    import sys
    n, k = map(int, sys.stdin.readline().split())
    arr = sys.stdin.readline().split()
    root = build_tree(arr)
    print(kthSmallest(root, k))

import java.io.*;
import java.util.*;

class TreeNode {
    int val;
    TreeNode left, right;
    TreeNode(int x) { val = x; }
}

public class Main {
    static int count, k, ans;

    // 根据层次遍历构造二叉树
    public static TreeNode buildTree(String[] tokens) {
        if (tokens.length == 0 || tokens[0].equals("null"))
            return null;
        
        TreeNode root = new TreeNode(Integer.parseInt(tokens[0]));
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int i = 1;
        while (i < tokens.length) {
            TreeNode current = queue.poll();
            // 构造左子节点
            if (i < tokens.length && !tokens[i].equals("null")) {
                current.left = new TreeNode(Integer.parseInt(tokens[i]));
                queue.offer(current.left);
            }
            i++;
            // 构造右子节点
            if (i < tokens.length && !tokens[i].equals("null")) {
                current.right = new TreeNode(Integer.parseInt(tokens[i]));
                queue.offer(current.right);
            }
            i++;
        }
        return root;
    }

    public static void inorder(TreeNode root) {
        if(root == null || ans != Integer.MIN_VALUE) return;
        inorder(root.left);
        if(++count == k) {
            ans = root.val;
            return;
        }
        inorder(root.right);
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] first = br.readLine().split(" ");
        int n = Integer.parseInt(first[0]);
        k = Integer.parseInt(first[1]);
        String[] arr = br.readLine().split(" ");
        TreeNode root = buildTree(arr);
        ans = Integer.MIN_VALUE;
        inorder(root);
        System.out.println(ans);
    }
}

#include <bits/stdc++.h>
using namespace std;

struct TreeNode {
    int val;
    TreeNode *left, *right;
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};

// 根据层次遍历结果构造二叉树
TreeNode* buildTree(const vector<string>& tokens) {
    if (tokens.empty() || tokens[0] == "null")
        return nullptr;
    
    TreeNode* root = new TreeNode(stoi(tokens[0]));
    queue<TreeNode*> q;
    q.push(root);
    int i = 1;
    while (i < tokens.size()) {
        TreeNode* current = q.front();
        q.pop();
        // 构造左子节点
        if (i < tokens.size() && tokens[i] != "null") {
            current->left = new TreeNode(stoi(tokens[i]));
            q.push(current->left);
        }
        i++;
        // 构造右子节点
        if (i < tokens.size() && tokens[i] != "null") {
            current->right = new TreeNode(stoi(tokens[i]));
            q.push(current->right);
        }
        i++;
    }
    return root;
}

int countK = 0, answer = -1;
void inorder(TreeNode* root, int k) {
    if(!root || answer != -1) return;
    inorder(root->left, k);
    if(++countK == k) {
        answer = root->val;
        return;
    }
    inorder(root->right, k);
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, k;
    cin >> n >> k;
    vector<string> arr(n);
    for(int i = 0; i < n; i++) cin >> arr[i];
    TreeNode* root = buildTree(arr);
    inorder(root, k);
    cout << answer << "\n";
    return 0;
}