二叉树自底向上层序遍历

利用递归的特性来实现：

class Solution {
public:
    /**
     *
     * @param root TreeNode类
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        // write code here
        vector<vector<int> > res;
        if (!root) return res;
        queue<TreeNode *> qu;
        qu.push(root);
        bottomToTop(qu, res);
        return res;
    }

    void bottomToTop(queue<TreeNode *> qu, vector<vector<int> > &res) {
        if (qu.empty()) return;
        vector<int> vec;
        int size = qu.size();
        for (int i = 0; i < size; ++i) {
            TreeNode *node = qu.front();
            qu.pop();
            vec.push_back(node->val);
            if (node->left) qu.push(node->left);
            if (node->right) qu.push(node->right);
        }
        bottomToTop(qu, res);
        res.push_back(vec);
    }
};