102.Binary Tree Level Order Traversal

1- queue - BFS

/**
 * 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>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            List<Integer> level = new ArrayList<>();
            int cnt = queue.size();
            for(int i = 0; i < cnt; i++){
                TreeNode node =queue.poll();
                level.add(node.val);
                if(node.left != null){
                    queue.add(node.left);
                }
                if(node.right != null){
                    queue.add(node.right);
                }
            }
            res.add(level);
        }
        return res;
    }
}