[clock 223] [binary tree] [leetcode high frequency]: 102 Sequence traversal of binary tree

1、 Title Description

Give you the root node of the binary tree root , Returns the Sequence traversal . （ That is, layer by layer , Access all nodes from left to right ）.

2、 Algorithm analysis

The level traversal of binary tree ：

Store nodes of each layer with queues , Traverse the queue , Add nodes in the queue to the result set , And judge whether the left and right nodes of the current node are empty , Add the results of each layer to the result set .

Queue operations ：

① Enter the team ：queue.offer();

② Out of the team ：queue.poll();

3、 Code implementation

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null){
            return res;
        } 
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> list = new ArrayList<>();
            int size = queue.size();
            for(int i = 0;i < size;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);
                }
            }
            res.add(list);
        }
        return res;
    }
}