【LeetCode】102.二叉树的层序遍历

题目https://leetcode.cn/problems/binary-tree-level-order-traversal/description/思路边界处理根节点为空则直接返回空列表队列初始化创建队列将根节点入队逐层遍历循环处理直到队列为空记录当前队列长度即当前层节点数创建当前层的值列表循环取出当前层所有节点节点值加入当前层列表左右子节点不为空加入队列供下一层使用将当前层列表加入结果集返回结果核心利用队列的先进先出特性通过记录每层节点数量来控制分层输出。code/** * 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; * } * } */classSolution{publicListListIntegerlevelOrder(TreeNoderoot){ListListIntegerresultnewLinkedList();DequeTreeNodedequenewLinkedList();if(rootnull)returnresult;deque.offer(root);//队不为空while(deque.isEmpty()false){ListIntegerlistnewArrayList();intnodeNumdeque.size();while(nodeNum0){TreeNodenodedeque.poll();nodeNum--;list.add(node.val);if(node.left!null){deque.offer(node.left);}if(node.right!null){deque.offer(node.right);}}result.add(list);}returnresult;}}