Leetcode: Binary Tree Zigzag Level Order Traversal

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本文深入探讨了如何使用Java编程语言实现二叉树的Zigzag层次遍历算法，通过栈数据结构巧妙地解决节点顺序问题，为读者提供了一个清晰的实现步骤和代码示例。

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its zigzag level order traversal as:

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

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
		// Start typing your Java solution below
		// DO NOT write main() function
		ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
		ArrayList<Integer> line;
		Stack<TreeNode> level = new Stack<TreeNode>();
		Stack<TreeNode> next_lev;
		if (root == null)
			return res;
		level.add(root);
		boolean flag = false;
		while (!level.isEmpty()) {
			line = new ArrayList<Integer>();
			next_lev = new Stack<TreeNode>();
			while(!level.isEmpty()) {
				TreeNode cur = level.pop();
				line.add(cur.val);
				if (flag) {
					if (cur.right != null)
						next_lev.add(cur.right);
					if (cur.left != null)
						next_lev.add(cur.left);
				} else {
					if (cur.left != null)
						next_lev.add(cur.left);
					if (cur.right != null)
						next_lev.add(cur.right);
				}
			}
			res.add(line);
			level = next_lev;
			flag = !flag;
		}
		return res;
	}
}