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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

分析

1. totally the same question as Binary Tree Level Order Traversal, except that due to zigzag level order, Collections.reverseis needed here

图解

解题

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        Boolean left_to_right = true;
        while(!q.isEmpty()) {
            int levelNodeNum = q.size();
            List<Integer> currLevel = new ArrayList<>();
            for(int i = 0; i < levelNodeNum; i++) {
                TreeNode currNode = q.poll();
                currLevel.add(currNode.val);
                if (currNode.left != null)
                    q.add(currNode.left);
                if (currNode.right != null)
                    q.add(currNode.right);
            }
            if(!left_to_right) Collections.reverse(currLevel);
            result.add(currLevel);
            left_to_right = !left_to_right;
        }
        return result;
    }
}