LeetCode 107. Binary Tree Level Order Traversal II（二叉树分层遍历）

原题网址：https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

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]
]

方法：广度优先搜索。

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> levels = new ArrayList<>();
if (root == null) return levels;
List<TreeNode> currents = new ArrayList<>();
currents.add(root);
while (!currents.isEmpty()) {
List<Integer> level = new ArrayList<>();
for(TreeNode current:currents) level.add(current.val);
levels.add(0, level);
List<TreeNode> nexts = new ArrayList<>();
for(TreeNode current:currents) {
if (current.left != null) nexts.add(current.left);
if (current.right != null) nexts.add(current.right);
}
currents = nexts;
}
return levels;
}
}