链接:
https://leetcode.com/problems/binary-tree-level-order-traversal/
大意:
层次遍历二叉树。要求按层次从左到右输出每层的节点值。例子:
思路:
层次遍历,使用两个变量ll以及l记录当前层的最后一个节点和下一层加入到队列中的元素
代码:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null)
return res;
LinkedList<TreeNode> queue = new LinkedList<>();
TreeNode l = root, ll = root; // l为当前加入到队列的最后一个元素 ll为上一层的最末尾元素
queue.addLast(root);
List<Integer> tmp = new ArrayList<>();
while (!queue.isEmpty()) {
TreeNode node = queue.removeFirst();
tmp.add(node.val);
if (node.left != null) {
queue.addLast(node.left);
l = node.left;
}
if (node.right != null) {
queue.addLast(node.right);
l = node.right;
}
if (node == ll) {
res.add(new ArrayList<>(tmp));
tmp = new ArrayList<>();
ll = l;
}
}
return res;
}
}
结果:
结论:
层次遍历的实现