Binary Tree Level Order Traversal II

Binary Tree Level Order Traversal II

问题：

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).

思路：

　　层次遍历 BFS 

我的代码：

public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> rst = new ArrayList<List<Integer>>();
        if(root == null) return rst;
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty())
        {
            int size = queue.size();
            List<Integer> list = new ArrayList<Integer>();
            for(int i = 0; i < size; i++)
            {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
            rst.add(list);
        }
        Collections.reverse(rst);
        return rst;
    }
}

他人代码：

public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

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

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        int currLevelNodeNum = 1;
        int nextLevelNodeNum = 0;

        while (currLevelNodeNum != 0) {
            ArrayList<Integer> currLevelResult = new ArrayList<Integer>();
            nextLevelNodeNum = 0;

            while (currLevelNodeNum != 0) {
                TreeNode node = queue.poll();

                currLevelNodeNum--;
                currLevelResult.add(node.val);

                if (node.left != null) {
                    queue.offer(node.left);
                    nextLevelNodeNum++;
                }

                if (node.right != null) {
                    queue.offer(node.right);
                    nextLevelNodeNum++;
                }
            }

            result.add(0, currLevelResult);
            currLevelNodeNum = nextLevelNodeNum;
        }
        return result;
    }
}

学习之处：

• ArrayList竟然还有这个add(index,Object)功能，如此便省去了翻转链表的时间，插入表头的时间O(n)，翻转链表的时间O(n)，实际的程序运行时间也一致。