102.二叉树的层序遍历

题目来源

/**
 * 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);//把节点加入队列尾部
        while(!queue.isEmpty()){
            int size = queue.size();//记录"当前层"有多少节点
            List<Integer> level = new ArrayList<>();
            for(int i=0;i<size;i++){
                TreeNode node = queue.poll();//取出队首元素，并删除。
                level.add(node.val);
                if(node.left != null){
                    queue.offer(node.left);
                }
                if(node.right != null){
                    queue.offer(node.right);
                }
            }
            res.add(level);
        }
        return res;
    }
}