二叉树的层序遍历II
题目描述:
给定一个二叉树,返回其节点值自底向上的层序遍历。 (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)
/**
* 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 {
private List<List<Integer>> result = new ArrayList<List<Integer>>();
public List<List<Integer>> levelOrderBottom(TreeNode root) {
helper(root,0);
return result;
}
public void helper(TreeNode root, int level) {
if (root == null) return;
if (result.size() <= level) result.add(0, new ArrayList<>());
result.get(result.size() - level - 1).add(root.val); // 倒数
helper(root.left, level + 1);
helper(root.right, level + 1);
}
}
由题意可知,利用先序遍历的递归方式,将元素从最底层开始依次记录到result中,这里要注意顺序的问题。详细请看代码,有疑问欢迎留言。