題目：

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]
]

題意及分析：寬度遍歷一棵樹。用隊列保存中間值即可。

代碼:

/**
 * 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<>();
        if(root==null) return res;
        List<Integer> list = new ArrayList<>();
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        Queue<TreeNode> queue1=new LinkedList<>();
        while(!queue.isEmpty()||!queue1.isEmpty()){
            queue1.clear();
            while(!queue.isEmpty()){        //遍歷一層
                TreeNode now = queue.poll();
                list.add(now.val);
                if(now.left!=null)
                    queue1.offer(now.left);
                if(now.right!=null) 
                    queue1.offer(now.right);
            }
            queue=new LinkedList<>(queue1);       //進行下一層的遍歷
            res.add(new ArrayList<>(list));
            list.clear();       //清空
        }
        return res;
    }
}

[LeetCode] 102. Binary Tree Level Order Traversal Java