LC.102. Binary Tree Level Order Traversal

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

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]
]
Corner Cases

What if the binary tree is null? Return an empty list of list in this case:
return new ArrayList<List<Integer>>();

 

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