103. Binary Tree Zigzag Level Order Traversal

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]
]

分析：用一个flag来表示存入list的方向即可。

public class Solution {
     public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> allLists = new ArrayList<>();
        if (root == null) return allLists;

        List<Integer> temp = new LinkedList<>();
        Queue<TreeNode> queue = new LinkedList<>();

        queue.offer(root);
        int direction = 1;

        while(!queue.isEmpty()) {
            int size = queue.size();

            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (direction == 1) {
                    temp.add(node.val);
                } else {
                    temp.add(0, node.val);
                }
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            allLists.add(new LinkedList<Integer>(temp));
            temp.clear();
            direction *= -1;
        }
        return allLists;
    }
}