LeetCode（八）107. Binary Tree Level Order Traversal II-优快云博客

本文链接：https://blog.youkuaiyun.com/u_hcy2000/article/details/84947991

本文介绍了一种二叉树的层次遍历算法，该算法从叶子节点开始，向上到根节点进行逆序层次遍历。通过使用队列和栈的数据结构，能够有效地实现这一遍历过程，最后返回一个逆序的节点值列表。

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问题描述：
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],

return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]

求解思路：对比102. Binary Tree Level Order Traversal
这里我再用一个栈来存储所有的有序状态下sublist，最后再从栈中pop出来，则形成逆序的list

代码如下：

class Solution {
	public class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}
	
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> list = new LinkedList<List<Integer>>();
        Stack<List<Integer>> stack = new Stack<List<Integer>>();
        if(root == null) {
        	return list;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        
        while(!queue.isEmpty()) {
        	List<Integer> sublist = new LinkedList<Integer>();
        	int size = queue.size();
        	for(int i = 0; i < size; i++) {
        		TreeNode currentNode = queue.poll();
        		sublist.add(currentNode.val);
        		if(currentNode.left != null) {
        			queue.add(currentNode.left);
        		}
        		if(currentNode.right != null) {
        			queue.add(currentNode.right);
        		}
        	}
        	stack.push(sublist);
        }
        
        while(!stack.isEmpty()) {
        	list.add(stack.pop());
        }
        return list;
    }
    
}