[LeetCode]Binary Tree Level Order Traversal II

最新推荐文章于 2021-11-18 16:57:58 发布

原创最新推荐文章于 2021-11-18 16:57:58 发布 · 567 阅读

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#Java #LeetCode #Breadth-first Search #Tree

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本文介绍了一种算法，用于实现二叉树的层次遍历并以从叶节点到根节点的方式返回各层级节点的值。通过递归方法，文章详细展示了如何构建一个解决方案来完成这一任务。

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,#,#,15,7},

return its bottom-up level order traversal as:

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

confused what "{1,#,2,3}" means?

Tree Breadth-first Search

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
	List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        levelOrderBottom(root,0);
        Collections.reverse(res);
        return res;
    }
    
    private void levelOrderBottom(TreeNode root, int level){
    	if(root == null) return;
    	if(level==res.size()){
    		List<Integer> list = new ArrayList<>();
    		res.add(list);
    	}
    	res.get(level).add(root.val);
    	levelOrderBottom(root.left,level+1);
    	levelOrderBottom(root.right,level+1);
    }
}