102. Binary Tree Level Order Traversal ------层序遍历

 

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]
]


层序遍历 利用queue

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.add(root);
        while (!queue.isEmpty()) {
            int levlnum = queue.size();
            List<Integer> res_temp = new ArrayList<Integer>();
            for (int i = 0;i<levlnum ;i++ ){ //把每层的左右节点都保存到queue里 
                //并讲当层的值从queue里弹出，加到res_temp 中
            if(queue.peek().left!=null) queue.add(queue.peek().left);
            if(queue.peek().right!=null) queue.add(queue.peek().right);

                res_temp.add(queue.poll().val);
            }
            res.add(res_temp);
        }
        return res;
    }
}