Binary Tree Level Order Traversal II

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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,#,#,15,7},

return its bottom-up level order traversal as:

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

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

/**
 * 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) {
        
        vector<vector<int> > retVtr;
        stack<vector<int>> vtrStack;
        
        list<TreeNode*> parentList;
        
        
        if (!root)
            return retVtr;
            
        parentList.push_back(root);
        
        while (parentList.size() > 0)
        {
            list<TreeNode*> curList;
            vector<int> valVector;
            while (parentList.size() > 0)
            {
                TreeNode *pCurNode = parentList.front();
                valVector.push_back(pCurNode->val);
                
                if (pCurNode->left)
                {
                    curList.push_back(pCurNode->left);
                }
                
                if (pCurNode->right)
                {
                    curList.push_back(pCurNode->right);
                }
                
                parentList.erase(parentList.begin());
            }
            
            vtrStack.push(valVector);
            parentList = curList;
        }
        
        
        while (!vtrStack.empty())
        {
            retVtr.push_back(vtrStack.top());
            vtrStack.pop();
        }
            
        return retVtr;
    }
};