Jianzhi offer 32-I. Print binary tree from top to bottom

Jianzhi offer 32-I. Print binary tree from top to bottom

Title description

Problem-solving ideas

Level traversal.

Note that to convert ArrayList to int[] array, you need to manually use a for loop.

class Solution {
    
    
    public int[] levelOrder(TreeNode root) {
    
    
        if (root == null) return new int[0];

        ArrayList<Integer> res = new ArrayList<>();

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
    
    
            TreeNode currNode = queue.poll();
            //保存结果
            res.add(currNode.val);
            //左右孩子入队
            if (currNode.left != null) queue.offer(currNode.left);
            if (currNode.right != null) queue.offer(currNode.right);
        }
        
        int[] resOfArray = new int[res.size()];
        for (int i = 0; i < resOfArray.length; i++) {
    
    
            resOfArray[i] = res.get(i);
        }
        return resOfArray;
    }
}