題目

給定一個二叉樹，返回其節點值自底向上的層次遍歷。 （即按從葉子節點所在層到根節點所在的層，逐層從左向右遍歷）
例如：

給定二叉樹 [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
返回其自底向上的層次遍歷為：
[
  [15,7],
  [9,20],
  [3]
]

/**
 * 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> > res;
        if(root == NULL)
            return res;
        queue < pair<TreeNode*, int> > q; //儲存節點和節點所在的層
        q.push(make_pair(root, 0));
        while
 
(!q.empty()){
            TreeNode* node = q.front().first;
            int level = q.front().second;
            q.pop();

            if(level == res.size())
               res.push_back(vector<int>() );

            res[level].push_back(node->val);
            if(node->left != NULL)
                 q.push(make_pair(node->left, level + 1
 
));
            if(node->right != NULL)
                 q.push(make_pair(node->right, level + 1));
        }
        vector<vector<int>> resBottom;
        for(int i = res.size() -1;i >= 0; i--){
            vector<int> tmp = res[i];
            resBottom.push_back(tmp);
        }
        return resBottom;
    }
};