[leetcode-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]
]

思路：

这个跟leetcode-102-Binary Tree Level Order Traversal 其实差不多的。

唯一不同的是在每一层遍历完之后，得到本层的数值，插入到最终result数组里的时候从头插入，即可实现倒序。

    vector<vector<int>> levelOrderBottom(TreeNode* root)
    {
        queue<TreeNode*>que;
        vector<vector<int>>result;
        if (root == NULL)return result;
        que.push(root);

        while (!que.empty())
        {
            int size = que.size();
            vector<int> temp;
            for (int i = 0; i < size; i++)
            {
                TreeNode* node = que.front();
                que.pop();
                temp.push_back(node->val);
                if (node->left != NULL)que.push(node->left);
                if (node->right != NULL)que.push(node->right);
            }            
            result.insert(result.begin(), temp);//从头部插入即可实现倒序
        }
        return result;
    }