Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example: Given binary tree [3,9,20,null,null,15,7],

3

/ \ 9 20 / \ 15 7

return its level order traversal as:

[ [3], [9,20], [15,7] ]

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
   public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();

        List<TreeNode> currentLevel = new ArrayList<>();
           if (root != null) {
            currentLevel.add(root);
        }

        while (!currentLevel.isEmpty()) {
            List<Integer> cur = currentLevel.stream().map(item -> item.val).collect(Collectors.toList());
            result.add(cur);            

            List<TreeNode> nextLevel = new ArrayList<>();
            currentLevel.forEach(
                    item -> {
                        if (item.left != null) {
                            nextLevel.add(item.left);
                        }

                        if (item.right != null) {
                            nextLevel.add(item.right);
                        }
                    }
            );

            currentLevel = nextLevel;
        }

        return result;
    }
}