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

本文链接：https://blog.youkuaiyun.com/rosepicker/article/details/73832107

本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的C++实现示例。

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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,null,null,15,7],

return its bottom-up level order traversal as:

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

代码如下：

/**
 * Definition for a binary tree node.
 * 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) {
        vector<vector<int>> result; 
        vector<vector<int>> tmp;
        vector<int> data;  
        if(root == NULL)  
            return result;  
          
        deque<TreeNode*> level;  
        level.push_back(root);  
        while(!level.empty())  
        {  
            data.clear();  
            int num = level.size();  
            for(int i=0;i<num;i++)  
            {  
                if(level.front()->left != NULL)  
                    level.push_back(level.front()->left);  
                if(level.front()->right != NULL)  
                    level.push_back(level.front()->right);  
                data.push_back(level.front()->val);  
                level.pop_front();  
            }  
            result.push_back(data);  
        }  
        for(int i=result.size()-1;i>=0;i--)
            tmp.push_back(result[i]);
        return tmp;  
    }
};