Binary Tree Level Order Traversal 2 二叉树层序遍历
给一个二叉树的root,返回其节点值从低向上遍历,每一层从左到右 遍历。
Input: root = [3,9,20,null,null,15,7]
Output: [[15,7],[9,20],[3]]
/**
* 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) {
List<List<Integer>> ans = new ArrayList<>();
if(root ==null){
return ans;
}
LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
while(!queue.isEmpty()){
List<Integer> arr = new ArrayList<>();
int length = queue.size();
for(int i=0;i<length;i++){
TreeNode tmp = queue.poll();
arr.add(tmp.val);
if(tmp.left!=null){
queue.add(tmp.left);
}
if(tmp.right!=null){
queue.add(tmp.right);
}
}
ans.add(arr);
}
Collections.reverse(ans);
// for(int i=0,j=ans.size()-1;i<j;i++,j--){
// List<Integer> tmp = ans.get(i);
// ans.set(i,ans.get(j));
// ans.set(j,tmp);
//
// }
return ans;
}
}
Tag
tree
BFS