[자료구조] Binary Tree Level Order Traversal

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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; }
 * }
 */
class Solution {
    List<List<Integer>> returnList = new ArrayList<List<Integer>>();
    ArrayList<TreeNode> queue = new ArrayList<TreeNode>();
    int pos = 0;
    int level = 0;
    int queueSize = 0;
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root != null){
            queue.add(root);    
        }
        while(pos != queue.size()){
            queueSize = queue.size();
            List<Integer> levelValueList = new ArrayList<Integer>();
            while(pos < queueSize){
                TreeNode node = queue.get(pos);
                if(node.left != null){
                    queue.add(node.left);
                }
                if(node.right != null){
                    queue.add(node.right);
                }
                levelValueList.add(node.val);
                pos++;
            }
            returnList.add(levelValueList);
        }
        return returnList;
    }
}