p103

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LeetCode p103 Binary Tree Zigzag Level Order Traversal 题解

1.题目：

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,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]

题意：

输入一二叉树，返回他每层元素的列表，奇数列从左到右，偶数列从右到左。

2.解题思路:

前续遍历

3.代码

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> ans = new ArrayList<List<Integer>>();

	public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
		zigzagLevelOrder(root, 0);
		return ans;
	}

	public void zigzagLevelOrder(TreeNode root, int count) {
    	if (root == null)
			return;
		
		if (ans.size() <= count) {
			ans.add(new ArrayList<Integer>());
		}
		List<Integer> a = ans.get(count);
		if(count % 2 == 0) a.add(root.val);
        else a.add(0, root.val);
		zigzagLevelOrder(root.left, count + 1);
		zigzagLevelOrder(root.right, count + 1);


	}
}

4.一些总结：