面试必刷TOP101：27、按之字形顺序打印二叉树

题目

题解

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param pRoot TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        // write code here
        ArrayList<ArrayList<Integer>> layers = new ArrayList<ArrayList<Integer>>();

        if (pRoot == null) {
            return layers;
        }

        ArrayList<Integer> layer = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();

        // Initialize first node
        layer.add(pRoot.val);
        layers.add(layer);
        queue.offer(pRoot);
        boolean reverse = false;

        while (!queue.isEmpty()) {
            // Reverse flag
            reverse = !reverse;
            layer = new ArrayList<Integer>();

            Stack<Integer> stack = new Stack<Integer>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();

                // Loop for `node.left` and `node.right`
                for (int j = 0; j < 2; j++) {
                    TreeNode current = j < 1 ? node.left : node.right;
                    if (current != null) {
                        // System.out.println(
                        //     node.val + (j < 1 ? ".left = " : ".right = ") + current.val
                        // );

                        if (!reverse) {
                            // Append to result in ascending order
                            layer.add(current.val);
                        } else {
                            // Reverse order for current layer
                            stack.push(current.val);
                        }

                        // Add node to queue for next layer
                        queue.offer(current);
                    }
                }
            }

            // Reverse needed for the current layer
            while (!stack.isEmpty()) {
                layer.add(stack.pop());
            }

            if (layer.size() > 0) {
                layers.add(layer);
            }
        }

        return layers;
    }
}