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]
]

将二叉树每一层的节点放入二维数组中

C++（6ms）：

/**
 * 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) {
        if (root == NULL)
            return vector<vector<int>>() ;
        vector<vector<int>> res ;
        queue<TreeNode*> que ;
        que.push(root) ;
        while(!que.empty()){
            int len = que.size() ;
            vector<int> vec ;
            for(int i = 0 ; i < len ; i++){
                TreeNode* node = que.front() ;
                que.pop() ;
                vec.push_back(node->val) ;
                if (node->left)
                    que.push(node->left) ;
                if (node->right)
                    que.push(node->right) ;
            }
            res.push_back(vec) ;
        }
        reverse(res.begin() , res.end()) ;
        return res ;
    }
};