题目
思路
正常层序遍历输出: [[3],[9,20],[15,7]]
这道题要求的输出:[[15,7],[9,20],[3]]
可以观察到,只要我们把原来的结果reverse一下就行了。
代码
//leetcode submit region begin(Prohibit modification and deletion)
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
* 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>> levelOrderBottom(TreeNode root) {
//创建一个辅助队列,存放节点
Queue<TreeNode> queue = new LinkedList<TreeNode>();
//创建一个结果List
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
queue.add(root);
while (!queue.isEmpty()) {
int len = queue.size();
List<Integer> item = new ArrayList<>();
while (len > 0) {
TreeNode temp = queue.poll();
item.add(temp.val);
if (temp.left != null)
queue.add(temp.left);
if (temp.right != null)
queue.add(temp.right);
len--;
}
res.add(item);
}
Collections.reverse(res);
return res;
}
}
//leetcode submit region end(Prohibit modification and deletion)