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。

/**
 * 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>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> allLists = new ArrayList<>();
        if (root == null) return allLists;
        
        List<Integer> temp = new LinkedList<>();
        List<TreeNode> list1 = new LinkedList<>();
        List<TreeNode> list2 = new LinkedList<>();
    
        list1.add(root);
        int direction = 1;
    
        while(list1.size() != 0) {
            for (TreeNode node : list1) {
                if (direction == 1) {
                    temp.add(node.val);
                } else {
                    temp.add(0, node.val);
                }
                if (node.left != null) list2.add(node.left);
                if (node.right != null) list2.add(node.right);
            } 
            allLists.add(new LinkedList<Integer>(temp));
            temp.clear();
            list1 = list2;
            list2 = new ArrayList<TreeNode>();
            direction *= -1;
        }
        return allLists;
    }
}