二叉树最近的叶节点

/*
 * Problem: Find the closest leaf node in a binary tree.
 * Approach: 
 * 1. Use a modified BFS approach utilizing Java Streams. 
 * 2. Traverse the tree level by level. 
 * 3. If a leaf node is found, return its value. 
 */

import java.util.*;
import java.util.stream.*;

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

public class ClosestLeafInBinaryTreeStream {
    public int findClosestLeaf(TreeNode root) {
        if (root == null) return -1;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<TreeNode> level = new ArrayList<>();
            while (!queue.isEmpty()) {
                level.add(queue.poll());
            }
            for (TreeNode node : level) {
                if (node.left == null && node.right == null) {
                    return node.val;  // Found a leaf node
                }
            }
            queue.addAll(level.stream()
                               .flatMap(n -> Stream.of(n.left, n.right))
                               .filter(Objects::nonNull)
                               .collect(Collectors.toList()));
        }
        return -1;  // In case no leaf node is found
    }
}