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