Day 56

题目描述

思路

锯齿形就是一层是从左向右，一层是从右向左，那么我们可以分析样例，对于第奇数层是从左向右，第偶数层是从右向左，于是可以采取一个计数器，采取链表方式，从左向右就是正常插入到链表后面，从右向左就是插入到链表前面。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>>result=new ArrayList<>();
        if(root==null){
            return result;
        }
        Queue<TreeNode>tree=new LinkedList<>();//保存树的节点
        int count;
        TreeNode x;
        int y=0;
        tree.offer(root);
        while(!tree.isEmpty()){
            count=tree.size();
            List<Integer>res=new LinkedList<>();
            for(int i=0;i<count;i++){
                x=tree.poll();
                if(x.left!=null) tree.offer(x.left);
                if(x.right!=null) tree.offer(x.right);
                if(y%2==0){
                    res.add(x.val);
                }
                else{
                    res.addFirst(x.val);
                }
            }
            y++;
            result.add(res);
        }
        return result;
    }
}