Binary Tree Level Order Traversal II-LeetCode-优快云博客

本文介绍了一种算法，该算法能够实现二叉树的层次遍历并按从叶节点到根节点的顺序返回各层级节点值。通过使用队列进行广度优先搜索，文章详细解释了如何收集每一层的数据，并在最后反转整个列表以达到题目要求。

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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).

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

return its bottom-up level order traversal as:

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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
       	queue <TreeNode *> m_t;
	    vector <int> lresult;
	    vector < vector <int> > result;
	    TreeNode *temp;
    	int lnum = 0;
    	if (root == NULL)
    	{
    		return result;
    	}
    	m_t.push(root);
    	while(!m_t.empty())
    	{
    		lnum = m_t.size();
	    	for (unsigned int i = 0; i < lnum; i++)
	    	{
	    		temp = m_t.front();
	    		lresult.push_back(temp->val);
	    		if (temp->left != NULL)
	    			m_t.push(temp->left);
    			if (temp->right != NULL)
    				m_t.push(temp->right);
    			m_t.pop();
    		}
    		result.push_back(lresult);
    		lresult.clear();
    	}//此时，输出结果为从根开始一直到叶子节点
    	vector < vector <int> > result1;//
    	for(int i = result.size() - 1; i >= 0; i--)
    	{
    		result1.push_back(result[i]); 
    	}
    	result.clear();
	return result1;//此时输出的结果为从叶子节点开始，一直到根
    }
};