【Leetcode】Binary Tree Level Order Traversal II

题目链接：https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

题目：

bottom-up level order

For example:

Given binary tree {3,9,20,#,#,15,7},

3 / \ 9 20 / \ 15 7

return its bottom-up level order traversal as:

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

思路：

自下而上的查找各层结点。

算法：

public List<List<Integer>> levelOrderBottom(TreeNode root) {
		int height = heightTree(root);
		List<List<Integer>> lists = new ArrayList<List<Integer>>();
		for (int i = height; i > 0; i--) { //自下而上
			List<Integer> list = new ArrayList<Integer>();
			list = kLevelNumber(root, 1, list, i);
			lists.add(list);
		}
		return lists;
	}

	public int heightTree(TreeNode p) {
		if (p == null)
			return 0;
		int h1 = heightTree(p.left);
		int h2 = heightTree(p.right);
		return h1 > h2 ? h1 + 1 : h2 + 1;
	}

	/**
	 * 返回kk层所有结点 
	 */
	public List<Integer> kLevelNumber(TreeNode p, int height, List<Integer> list, int kk) {
		if (p != null) {
			if (height == kk) {
				list.add(p.val);
			}
			list = kLevelNumber(p.left, height + 1, list, kk);
			list = kLevelNumber(p.right, height + 1, list, kk);
		}
		return list;
	}