【Leetcode】Binary Tree Level Order Traversal II

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

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,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

思路：

定义一个方法 根据传入的层数返回该层所有结点。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;}