題目：從上到下按層列印二叉樹，同一層的結點按從左到右的順序列印，每一層列印一行。

解題思路

　　用一個佇列來儲存將要列印的結點。為了把二叉樹的每一行單獨列印到一行裡，我們需要兩個變數：一個變量表示在當前的層中還沒有列印的結點數，另一個變量表示下一次結點的數目。

結點定義

private static class BinaryTreeNode {
    private int val;
    private BinaryTreeNode left;
    private BinaryTreeNode right;

    public BinaryTreeNode
 
() {
    }

    public BinaryTreeNode(int val) {
        this.val = val;
    }

    @Override
    public String toString() {
        return val + "";
    }
}

程式碼實現

import java.util.LinkedList;
import java.util.List;

public class Test60 {
    private static class BinaryTreeNode {
        private
 
 int val;
        private BinaryTreeNode left;
        private BinaryTreeNode right;

        public BinaryTreeNode() {
        }

        public BinaryTreeNode(int val) {
            this.val = val;
        }

        @Override
        public String toString() {
            return val + "";
        }
    }

    /**
     * 題目：從上到下按層列印二叉樹，同一層的結點按從左到右的順序列印，每一層列印一行。
     * @param
 
 root
     */
    public static void print(BinaryTreeNode root) {
        if (root == null) {
            return;
        }

        List<BinaryTreeNode> list = new LinkedList<>();
        BinaryTreeNode node;
        // 當前層的結點個數
        int current = 1;
        // 記錄下一層的結點個數
        int next = 0;
        list.add(root);

        while (list.size() > 0) {
            node = list.remove(0);
            current--;
            System.out.printf("%-3d", node.val);

            if (node.left != null) {
                list.add(node.left);
                next++;
            }
            if (node.right != null) {
                list.add(node.right);
                next++;
            }

            if (current ==0) {
                System.out.println();
                current = next;
                next = 0;
            }
        }
    }

    public static void main(String[] args) {
        BinaryTreeNode n1 = new BinaryTreeNode(1);
        BinaryTreeNode n2 = new BinaryTreeNode(2);
        BinaryTreeNode n3 = new BinaryTreeNode(3);
        BinaryTreeNode n4 = new BinaryTreeNode(4);
        BinaryTreeNode n5 = new BinaryTreeNode(5);
        BinaryTreeNode n6 = new BinaryTreeNode(6);
        BinaryTreeNode n7 = new BinaryTreeNode(7);
        BinaryTreeNode n8 = new BinaryTreeNode(8);
        BinaryTreeNode n9 = new BinaryTreeNode(9);

        n1.left = n2;
        n1.right = n3;
        n2.left = n4;
        n2.right = n5;
        n3.left = n6;
        n3.right = n7;
        n4.left = n8;
        n4.right = n9;

        print(n1);

    }
}

執行結果

說明