题目

• 原题连接：103. 二叉树的锯齿形层序遍历

1- 思路

• 二叉树的层序遍历，遇到奇数时，利用 Collections.reverse() 翻转即可

2- 实现

⭐103. 二叉树的锯齿形层序遍历——题解思路

class Solution {
    public List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        
        return Traversal(root);
    }

    public List<List<Integer>> Traversal(TreeNode root){
        if(root==null){
            return res;
        }
        // 借助 queue
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        // queue 不空
        int count = 0;
        while(!queue.isEmpty()){
            int len = queue.size();
            List<Integer> path = new ArrayList<>();
            while(len>0){
                TreeNode node = queue.poll();
                path.add(node.val);
                if(node.left!=null){
                    queue.offer(node.left);
                }
                if(node.right!=null){
                    queue.offer(node.right);
                }
                len--;
            }
            count++;
            if(count%2==1){
                res.add(new ArrayList(path));
            }else{
                Collections.reverse(path);
                res.add(new ArrayList(path));
            }
        }
        return res;
    }
}

2- ACM实现

public class levelTraversal {

    static class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(){}
        TreeNode(int x){
            val = x;
        }
    }

    public static TreeNode build(Integer[] nums){
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(nums[0]);
        queue.offer(root);
        int index = 1;
        while(!queue.isEmpty() && index<nums.length){
            TreeNode node = queue.poll();
            if(nums[index]!=null && index<nums.length){
                node.left = new TreeNode(nums[index]);
                queue.offer(node.left);
            }
            index++;
            if(nums[index]!=null && index<nums.length){
                node.right = new TreeNode(nums[index]);
                queue.offer(node.right);
            }
            index++;
        }
        return root;
    }

    static List<List<Integer>> res =new ArrayList<>();
    public static List<List<Integer>> levelTraversal(TreeNode root){
        if(root==null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int level = 0;
        while(!queue.isEmpty()){
            List<Integer> iterm = new ArrayList<>();
            int len = queue.size();
            while(len>0){
                TreeNode node = queue.poll();
                iterm.add(node.val);
                if(node.left!=null){
                    queue.offer(node.left);
                }
                if(node.right!=null){
                    queue.offer(node.right);
                }
                len--;
            }
            if(level%2==1) {
                Collections.reverse(iterm);
            }
            res.add(new ArrayList<>(iterm));
        }
        return res;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();
        input = input.replace("[","");
        input = input.replace("]","");
        String[] parts = input.split(",");
        Integer[] nums = new Integer[parts.length];
        for(int i = 0 ; i < parts.length ;i++){
            if(!parts[i].equals("null")){
                nums[i] = Integer.parseInt(parts[i]);
            }else{
                nums[i] = null;
            }
        }
        TreeNode root = build(nums);
        levelTraversal(root);
        System.out.println("结果为"+res.toString());
    }
}