二叉树广度优先搜索（BFS）- Java

/**
 * 广度优先搜索
 */
public class BFDMain {

    public static void main(String[] args) {

        TreeNode n11 = new TreeNode(11);
        TreeNode n10 = new TreeNode(10);
        TreeNode n9 = new TreeNode(9);
        TreeNode n8 = new TreeNode(8);
        TreeNode n7 = new TreeNode(7, n11, null);
        TreeNode n6 = new TreeNode(6, n9, n10);
        TreeNode n5 = new TreeNode(5);
        TreeNode n4 = new TreeNode(4, null, n8);
        TreeNode n3 = new TreeNode(3, n6, n7);
        TreeNode n2 = new TreeNode(2, n4, n5);
        TreeNode n1 = new TreeNode(1, n2, n3);

        System.out.println(bfs(n1));	// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
    }

    public static List<Integer> bfs(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();

        if (root == null) {
            return list;
        }

        queue.offer(root);

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            // 左子树不为空
            if (node.left != null) {
                queue.offer(node.left);
            }
            // 右子树不为空
            if (node.right != null) {
                queue.offer(node.right);
            }

            list.add(node.val);
        }

        return list;
    }

}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

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

    TreeNode(int val) {
        this.val = val;
    }

    @Override
    public String toString() {
        return "TreeNode{" +
                "val=" + val +
                ", left=" + left +
                ", right=" + right +
                '}';
    }
}