二叉树的层次遍历

class TreeNode {
    int value;
    TreeNode left;
    TreeNode right;

    public TreeNode(int value) {
        this.value = value;
        this.left = null;
        this.right = null;
    }
}

public class BinaryTreeLevelOrderTraversal {
    public List> levelOrderTraversal(TreeNode root) {
        List> result = new ArrayList<>();
        if (root == null) {
            return result;
        }

        Queue queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            List levelValues = new ArrayList<>();
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                levelValues.add(node.value);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            result.add(levelValues);
        }

        return result;
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);

        BinaryTreeLevelOrderTraversal solution = new BinaryTreeLevelOrderTraversal();
        List> traversalResult = solution.levelOrderTraversal(root);
        System.out.println(traversalResult);  // 输出：[[1], [2, 3], [4, 5, 6, 7]]
    }
}