【剑指offer】面试题32（1）：从上往下不分行打印二叉树

柔情只为你懂 2022-06-11 01:16 106阅读 0赞

## 完整代码地址 ##

[完整代码地址][Link 1]

## 题目 ##

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

## 思路 ##

很直观，用队列解决

## 代码 ##

public class _32_01_PrintTreeFromTopToBottom {
    
        public static class TreeNode {
            public int val = 0;
            public TreeNode left = null;
            public TreeNode right = null;
            public TreeNode(int val) {
                this.val = val;
            }
        }
    
        public static ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
            if(root == null)
                return new ArrayList<>();
    
            ArrayList<Integer> list = new ArrayList<>();
            Queue<TreeNode> queue = new LinkedList<>();
            queue.add(root);
    
            while(!queue.isEmpty()) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null)
                    queue.add(node.left);
                if(node.right != null)
                    queue.add(node.right);
            }
            return list;
        }
    }

## 测试 ##

public class _32_01_Test {
        
    
        public static void main(String[] args) {
            test1();
            test2();
            test3();
        }
    
        /**
         * 功能测试
         *     1
         *    / \
         *   2   3
         *  / \   \
         * 4   5   6
         */
        private static void test1() {
            TreeNode root = new TreeNode(1);
            TreeNode node2 = new TreeNode(2);
            TreeNode node3 = new TreeNode(3);
            TreeNode node4 = new TreeNode(4);
            TreeNode node5 = new TreeNode(5);
            TreeNode node6 = new TreeNode(6);
            root.left = node2;
            root.right = node3;
            node2.left = node4;
            node2.right = node5;
            node3.right = node6;
            System.out.println(_32_01_PrintTreeFromTopToBottom.PrintFromTopToBottom(root));
        }
    
        /**
         * 边界测试
         * 1.只有一个节点
         * 2.每个节点都只有左子节点
         * 3.每个节点都只有右子节点
         */
        private static void test2() {
            TreeNode root = new TreeNode(1);
    
            System.out.println(_32_01_PrintTreeFromTopToBottom.PrintFromTopToBottom(root));
    
            TreeNode node2 = new TreeNode(2);
            TreeNode node3 = new TreeNode(3);
            root.left = node2;
            node2.left = node3;
    
            System.out.println(_32_01_PrintTreeFromTopToBottom.PrintFromTopToBottom(root));
    
            root.left = null;
            node2.left = null;
            root.right = node2;
            node2.right = node3;
            System.out.println(_32_01_PrintTreeFromTopToBottom.PrintFromTopToBottom(root));
        }
    
        /**
         * 极端测试 
         * null
         */
        private static void test3() {
            System.out.println(_32_01_PrintTreeFromTopToBottom.PrintFromTopToBottom(null));
        }
    
    }

[Link 1]: https://github.com/linpeiyou/CodingInterviews