107. Binary Tree Level Order Traversal II

最新推荐文章于 2021-08-10 13:02:56 发布

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#LeetCode #Binary Tree Level Or #JAVA

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本文介绍了一种解决二叉树节点值从叶到根、由左至右按层次遍历的问题的方法。通过递归算法实现，先确定当前层级再递归地处理下一层级，最终返回一个列表包含所有层级的节点值。

1 题目

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

2代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private int len = 0;

	public List<List<Integer>> levelOrderBottom(TreeNode root) {
		List<List<Integer>> res = new ArrayList<List<Integer>>();
		if (root != null) {
			next(0, root, res);
		}
		return res;
	}

	public void next(int num, TreeNode treeNode, List<List<Integer>> res) {
		if (len <= num) {
			res.add(0, new LinkedList<>());
			len = res.size();
		}
		res.get(len - 1 - num).add(treeNode.val);

		if (treeNode.left != null) {
			next(num + 1, treeNode.left, res);
		}
		if (treeNode.right != null) {
			next(num + 1, treeNode.right, res);
		}
	}
}