题解 | #按之字形顺序打印二叉树#

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 {
    ArrayList<ArrayList<Integer>> res = new ArrayList<>();

    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        if (pRoot == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        boolean isOdd = true;//奇数从左往右遍历
        while (!queue.isEmpty()) {
            int n = queue.size();
            ArrayList<Integer> list = new ArrayList<>();
            if (isOdd) {
                //从左往右
                for (int i = 0; i < n; i++) {
                    TreeNode cur = queue.poll();
                    list.add(cur.val);
                    if (cur.left != null) {
                        queue.add(cur.left);
                    }
                    if (cur.right != null) {
                        queue.add(cur.right);
                    }
                }
                isOdd = false;
            } else {
                //从右往左
                for (int i = 0; i < n; i++) {
                    TreeNode cur = queue.pollLast();
                    list.add(cur.val);
                    if (cur.right != null) {
                        queue.addFirst(cur.right);
                    }
                    if (cur.left != null) {
                        queue.addFirst(cur.left);
                    }
                }
                isOdd = true;
            }
            res.add(list);
        }
        return res;
    }

}

思路二： 正常层序遍历，奇数不变，偶数行反转，用reverse函数。