（待思考--不采用逆置等的思想）107. Binary Tree Level Order Traversal II

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107. Binary Tree Level Order Traversal II

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],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

方法一、递归的方法：用到c++的反向迭代器

vector<vector<int> > result;    void levelTra(TreeNode *root, int level)    {        if(root == NULL)            return;        if(level == result.size())        {            vector<int> v;            result.push_back(v);        }        result[level].push_back(root->val);        levelTra(root->left, level+1);        levelTra(root->right, level+1);    }    vector<vector<int> > levelOrderBottom(TreeNode *root)     {        levelTra(root, 0);        return vector<vector<int> >(result.rbegin(), result.rend());    }

方法二、非递归的方法，层次遍历

（1）stack

vector<vector<int>> levelOrderBottom(TreeNode* root) {        vector<vector<int>> result;        if(NULL == root)            return vector<vector<int>>();                queue<TreeNode*> que;        que.push(root);        int countLevNodes = 1;        vector<int> temp;        stack<vector<int>> tempSta;        while(!que.empty())        {            TreeNode* tempTreeNode = que.front();            que.pop();            temp.push_back(tempTreeNode->val);            countLevNodes--;                        if(tempTreeNode->left != NULL)                que.push(tempTreeNode->left);            if(tempTreeNode->right != NULL)                que.push(tempTreeNode->right);            if(countLevNodes == 0)            {                tempSta.push(temp);                temp.clear();                countLevNodes = que.size();            }        }                while(!tempSta.empty())        {            temp = tempSta.top();            tempSta.pop();            result.push_back(temp);        }                return result;    }

（2）reverse

vector<vector<int>> levelOrderBottom(TreeNode* root) {        vector<vector<int>> result;        if(NULL == root)            return vector<vector<int>>();                queue<TreeNode*> que;        que.push(root);        int countLevNodes = 1;        vector<int> temp;        while(!que.empty())        {            TreeNode* tempTreeNode = que.front();            que.pop();            temp.push_back(tempTreeNode->val);            countLevNodes--;                        if(tempTreeNode->left != NULL)                que.push(tempTreeNode->left);            if(tempTreeNode->right != NULL)                que.push(tempTreeNode->right);            if(countLevNodes == 0)            {                result.push_back(temp);                temp.clear();                countLevNodes = que.size();            }        }                reverse(result.begin(),result.end());        return result;    }

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