107 Binary Tree Level Order Traversal II-优快云博客

本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的例子来展示如何实现这一遍历过程。

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]
]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public List<List<Integer>> levelOrderBottom(TreeNode root) {
    List<List<Integer>> list = new LinkedList<List<Integer>>();
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    if(root == null) { return list; }
    queue.offer(root);
    while(!queue.isEmpty()){
      int num = queue.size();
      List<Integer> levelList = new LinkedList<Integer>();
      for(int i = 0; i < num; i++){
        if(queue.peek().left != null)
          queue.offer(queue.peek().left);
        if(queue.peek().right != null)
          queue.offer(queue.peek().right);
        levelList.add(queue.poll().val);
      }
      list.add(0, levelList);
    }
    return list;
  }
}