[LeetCode] Binary Tree Level Order Traversal

最新推荐文章于 2021-11-17 20:57:12 发布

原创最新推荐文章于 2021-11-17 20:57:12 发布 · 556 阅读

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#LeetCode #BFS #Binary Tree

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本文详细解析了二叉树层次遍历的两种实现方式：递归和队列辅助，通过实例代码深入理解层次遍历的概念及其实现细节。

Total Accepted: 12041 Total Submissions: 39526

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its level order traversal as:

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>>   list = new ArrayList<ArrayList<Integer>>();

        preTraverse(root, list, 1);
        
        return list;
    }
    
    public void preTraverse(TreeNode root, ArrayList<ArrayList<Integer>> list, 
                                int level) {
        if (root == null) return;
        
        if (list.size() < level) list.add(new ArrayList<Integer>());
        list.get(level - 1).add(root.val);
        
        preTraverse(root.left, list, level + 1);
        preTraverse(root.right, list, level + 1);
    }
}

public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>>   list = new ArrayList<ArrayList<Integer>>();
        Queue<TreeNode>                 queue = new LinkedList<TreeNode>();
        ArrayList<Integer>              levelList = new ArrayList<Integer>();
        if (root == null) return list;
        
        queue.add(root);
        queue.add(null);
        while (!queue.isEmpty() && queue.peek() != null) {
            TreeNode top = queue.remove();
            levelList.add(top.val);
            
            if (top.left    != null) queue.add(top.left);
            if (top.right   != null) queue.add(top.right);
            
            if (queue.peek() == null) {
                queue.remove();
                queue.add(null);
                list.add(new ArrayList<Integer>(levelList));
                levelList.clear();
            }
        }
        return list;
    }
}