[leetcode] 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,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

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

https://oj.leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

思路：跟层序遍历类似，用queue来做，用null分隔每一层，然后

public class Solution {
   

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {

        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root == null)
            return res;

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        queue.add(null);
        boolean odd = true;
        List<Integer> tmp = new ArrayList<Integer>();
        while (!queue.isEmpty()) {
            TreeNode node = queue.remove();
            if (node == null) {
                if (odd)
                    res.add(new ArrayList<Integer>(tmp));
                else {
                    Collections.reverse(tmp);
                    res.add(new ArrayList<Integer>(tmp));
                }
                tmp.clear();
                odd = !odd;
                if (!queue.isEmpty())
                    queue.add(null);
            } else {
                // System.out.println(node.val);
                tmp.add(node.val);
                if (node.left != null)
                    queue.add(node.left);
                if (node.right != null)
                    queue.add(node.right);

            }

        }
        return res;

    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(9);
        root.right = new TreeNode(20);
        root.right.left = new TreeNode(15);
        root.right.right = new TreeNode(7);

        System.out.println(new Solution().zigzagLevelOrder(root));

    }

}

思路：分层的做法不变，可采用计数法。增加一个额外的flag用于翻转构建每层List的顺序。 LinkedList可以addFirst(), addLast(), removeFirst(), removeLast().

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null){
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        boolean left2Right = true;
        while(!queue.isEmpty()){
            int levelNum = queue.size();
            LinkedList<Integer> list = new LinkedList();
            for(int i=0;i<levelNum;i++){
                TreeNode out = queue.remove();
                if(left2Right){
                    list.addLast(out.val);
                }else{
                    list.addFirst(out.val);
                }
                
                if(out.left!=null){
                    queue.add(out.left);
                }
                if(out.right!=null){
                    queue.add(out.right);
                }
                
            }
            left2Right = !left2Right;
            res.add(list);
        }
        
        return res;
        
    }
}