之子形打印二叉树

import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
/*
    从返回值我们得知每层都要存一个ArrayList<Integer>
    然后再向ArrayList<ArrayList<Integer>>中存入每一层的ArrayList<Integer>
*/
public class Solution {

public static ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        int layer = 1;
        //s1存奇数层节点
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        s1.push(pRoot);
        //s2存偶数层节点
        Stack<TreeNode> s2 = new Stack<TreeNode>();
         
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
         
        while (!s1.empty() || !s2.empty()) {
            if (layer%2 != 0) {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s1.empty()) {
                    TreeNode node = s1.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s2.push(node.left);
                        s2.push(node.right);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            } else {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s2.empty()) {
                    TreeNode node = s2.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s1.push(node.right);
                        s1.push(node.left);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            }
        }
        return list;
    }
}