102. Binary Tree Level Order Traversal

문제

https://leetcode.com/problems/binary-tree-level-order-traversal/description/

풀이

class Solution {
    List<List<Integer>> list = new ArrayList<>();
    int index = 0;
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root == null) {
            return list ;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()) {
            int size = queue.size();
            list.add(new ArrayList<>());
            for(int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                list.get(index).add(node.val);
                if(node.left != null) {
                    queue.add(node.left);
                }
                if(node.right != null) {
                    queue.add(node.right);
                }
            }
            index++;
        }
        return list;
    }
}