107. Binary Tree Level Order Traversal II

1 題目

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],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

2代碼

/**
 * 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);
		}
	}
}