题解 | #按之字形顺序打印二叉树#

import java.util.ArrayList;
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 {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer> > ans = new ArrayList< >();
        if (pRoot == null) {
            return ans;
        }
        Queue<TreeNode> q1 = new LinkedList<>();
        q1.add(pRoot);
        //和普通的层序遍历区别就在于多了个flag
        boolean left_flag = true;
        while (!q1.isEmpty()) {
            int size = q1.size();
            ArrayList<Integer> curL = new ArrayList<>();

            for (int i = 0; i < size; i++) {

                TreeNode tmp = q1.poll();
                if (left_flag) {
                    curL.add(tmp.val);

                } else {
                    curL.add(0, tmp.val);

                }

                if (tmp.left != null) {
                    q1.add(tmp.left);

                }

                if (tmp.right != null) {
                    q1.add(tmp.right);

                }

            }

            ans.add(curL);

            left_flag = !left_flag;

        }


        return ans;
    }

}