[leetCode]103. 二叉树的锯齿形层序遍历

题目

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

给定一个二叉树，返回其节点值的锯齿形层序遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

例如：

给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回锯齿形层序遍历如下：

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

层序遍历 BFS

添加每一层的节点使用双端队列

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private List<List<Integer>> res = new ArrayList<>();
    
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if (root == null) return res;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean flag = false;
        while (!queue.isEmpty()) {
            int size = queue.size();
            LinkedList<Integer> level = new LinkedList<>();
            for (int  i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                if(!flag) {
                    level.offerLast(cur.val);
                } else {
                    level.offerFirst(cur.val);
                }
                if (cur.left != null) 
                    queue.offer(cur.left);
                if (cur.right != null) 
                    queue.offer(cur.right);
            }
            res.add(level);
            flag = !flag;
        }
        return res;
    }
}