Binary Tree Level Order Traversal II

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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).

描述

层序遍历，自底向上

分析

自顶向下遍历，将新的一层插入到最前面，lists.add(0, arrayList)

代码

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/**
 * 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>> lists = new ArrayList<List<Integer>>();

        if (root == null) {
            return lists;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);

        int count = 0, n = 1;

        while (!queue.isEmpty()) {

            ArrayList arrayList = new ArrayList<Integer>();

            while ((--n) >= 0) {

                TreeNode treeNode = queue.poll();
                arrayList.add(treeNode.val);

                if (treeNode.left != null) {
                    queue.offer(treeNode.left);
                    count++;
                }

                if (treeNode.right != null) {
                    queue.offer(treeNode.right);
                    count++;
                }
            }

            n = count;
            count = 0;
            lists.add(0, arrayList);      //lists.add(arrayList);（自顶向下）
        }

        return lists;
    }
}