103 - 二叉树的锯齿形层次遍历

题目描述

给定一个二叉树，返回其节点值的锯齿形层次遍历。（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],

返回锯齿形层次遍历如下：

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

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

题解

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if(root != null) {
            LinkedList<TreeNode> nodeQueue = new LinkedList<>();
            nodeQueue.add(root);
            help(result, nodeQueue, 1);
        }
        return result;
    }

    private void help(List<List<Integer>> result, LinkedList<TreeNode> nodeQueue, int level) {
        if(nodeQueue.size() == 0) {
            return;
        }
        LinkedList<TreeNode> nextNodeQueue = new LinkedList<>();
        List<Integer> vals = new ArrayList<>();
        while(nodeQueue.size() > 0) {
            TreeNode node = nodeQueue.pollFirst();
            vals.add(node.val);
            if(level % 2 == 1) {
                if(node.left != null) {
                    nextNodeQueue.addFirst(node.left);
                }
                if(node.right != null) {
                    nextNodeQueue.addFirst(node.right);
                }
            } else {
                if(node.right != null) {
                    nextNodeQueue.addFirst(node.right);
                }
                if(node.left != null) {
                    nextNodeQueue.addFirst(node.left);
                }
            }
        }
        result.add(vals);
        help(result, nextNodeQueue, level + 1);
    }
}