[leetcode]107. 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,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

广度优先遍历

/** * 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) {        Queue<TreeNode> queue=new LinkedList<>() ;List<List<Integer>> res =new LinkedList<>();if(root==null){    return res;}queue.offer(root);while(!queue.isEmpty()){        List<Integer> temp=new LinkedList<>();    int num = queue.size();    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);    }     temp.add(queue.poll().val);    }res.add(0, temp);}return res;    }}