leetcode-hot-100-20230928

102. 二叉树的层序遍（Java实现）

/**
 * 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) {
        
        //定义保存结果的result list
        List<List<Integer>> result = new ArrayList<>();
        
        if(root == null){
            return result;
        }
        
        Queue<TreeNode> que = new LinkedList<>();
        que.offer(root);

        while(!que.isEmpty()){
            //保存本层的tmp数组list
            List<Integer> tmpList = new ArrayList<>();
            
            int len = que.size();
            
            for(int i = 0;i < len;i++){
                TreeNode tmpNode = que.poll();
                tmpList.add(tmpNode.val);
                if(tmpNode.left != null){
                    que.offer(tmpNode.left);
                }
                if(tmpNode.right != null){
                    que.offer(tmpNode.right);
                }
            }
            
            result.add(tmpList);
        }
        return result;
    }
}