Binary Tree Level Order Traversa

原题链接

description

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]
]

思路分析

• 这里是层次遍历，层次遍历应该使用先进先去的队列数据结构，这里还需要记录每层的节点数目。

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