给定一个二叉树,返回其节点值自底向上的层次遍历。 (即按从叶子节点所在层到根节点所在的层,逐层从左向右遍历)
例如:
给定二叉树 [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
返回其自底向上的层次遍历为:
[
[15,7],
[9,20],
[3]
]
class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if(root == null)
return res;
Stack<List<Integer>> s = new Stack<>(); //利用栈
Queue<TreeNode> q = new LinkedList<>();
q.offer(root);
while(!q.isEmpty()) {
List<Integer> list = new ArrayList<>();
int count = q.size();
while(count-- > 0) {
TreeNode cur = q.poll();
list.add(cur.val);
if(cur.left != null)
q.offer(cur.left);
if(cur.right != null)
q.offer(cur.right);
}
s.push(list);
}
while(!s.isEmpty()) {
res.add(s.pop());
}
return res;
}
}