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  • Leetcode 107. Binary Tree Level Order Traversal II

    题目链接

    https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/

    题目描述

    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]
    ]
    

    题解

    层次遍历,用队列存起来。

    代码

    /**
     * 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<>();
            if (root == null) { return list; }
            Queue<TreeNode> queue = new LinkedList<>();
            queue.offer(root);
            
            while (!queue.isEmpty()) {
                int size = queue.size();
                List<Integer> subList = new LinkedList<>();
                for (int i = 0; i <size; i++) {
                    if (queue.peek().left != null) { queue.offer(queue.peek().left); }
                    if (queue.peek().right != null) { queue.offer(queue.peek().right); }
                    subList.add(queue.poll().val);
                }
                list.add(0, subList);
            }
            return list;
        }
    }
    
    
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  • 原文地址:https://www.cnblogs.com/xiagnming/p/9596746.html
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