Leetcode no. 102

102. 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]
]

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