102. 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]]

BST遍历，ArrayList存储，从上到下把每层元素加到ArrayList里，代码如下：

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

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