层数最深叶子节点的和

/*
 * 思路：
 * 使用Java Stream API的方式不太适合处理树的层序遍历，
 * 因为Stream API更适合处理集合类的数据。
 * 这里仍然使用BFS算法，但通过Stream API来构造和处理数据。
 */
import java.util.*;
import java.util.stream.Collectors;

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

public class Solution {
    public int deepestLeavesSum(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int sum = 0;
        while (!queue.isEmpty()) {
            List<TreeNode> currentLevel = new ArrayList<>();
            sum = 0; // reset sum for current level
            queue.stream().forEach(node -> {
                sum += node.val;
                if (node.left != null) currentLevel.add(node.left);
                if (node.right != null) currentLevel.add(node.right);
            });
            queue = new LinkedList<>(currentLevel);
        }
        return sum;
    }
}