剑指Offer32-III.从下到下打印二叉树

剑指 Offer 32 - III. 从上到下打印二叉树 III

Difficulty: 中等

请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，第三行再按照从左到右的顺序打印，其他行以此类推。

例如:
给定二叉树: [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

返回其层次遍历结果：

[
  [3],
  [20,9],
  [15,7]
]

提示：

1. 节点总数 <= 1000

Solution

Language: java

​/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root == null) return new ArrayList<>();
        List<List<Integer>> res = new ArrayList<>();
        Deque<TreeNode> cur1 = new ArrayDeque<>();
        Deque<TreeNode> cur2 = new ArrayDeque<>();
        Deque<TreeNode>  p = cur1;
        cur1.push(root);
        List<Integer> list = new ArrayList<>();
        while(!p.isEmpty()){
            TreeNode t = p.pop();
            list.add(t.val);
            if(p == cur1){
                if(t.left != null) cur2.push(t.left);
                if(t.right != null) cur2.push(t.right);
            }
            if(p == cur2){
                if(t.right != null) cur1.push(t.right);
                if(t.left != null) cur1.push(t.left);
            }
            if(p.isEmpty()){
                res.add(new ArrayList<>(list));
                list = new ArrayList<>();
                p = p == cur1 ? cur2 : cur1;
            }
        }
        return res;
    }
}