LeetCode - Binary Tree Zigzag Level Order Traversal

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,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

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

思路：

采用Binary Tree Level Order Traversal的方法，然后对第二行、第四行进行reverse。

package treetraversal;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class BinaryTreeZigzagLevelOrderTraversal {

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
//List<Integer> record = new ArrayList<Integer>();
levelOrderTraversal(res, root, 0);
for (int i = 0; i < res.size(); ++i) {
if (i % 2 == 1) {
Collections.reverse(res.get(i));
}
}
return res;
}

private void levelOrderTraversal(List<List<Integer>> res, TreeNode root, int k) {
if (root == null) return;
if (res.size() < k + 1) {
List<Integer> record = new ArrayList<Integer>();
record.add(root.val);
res.add(record);
} else {
res.get(k).add(root.val);
}
levelOrderTraversal(res, root.left, k + 1);
levelOrderTraversal(res, root.right, k + 1);
}

}