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]
]

Tips:

BFS

Code：

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