Leetcode: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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

 

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]
分析：代码如下：

class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > result;
        if(root == NULL) return result;
        
        vector<vector<TreeNode *> > levels;
        vector<TreeNode *> prev(1, root);
        
        while(!prev.empty()){
            levels.push_back(prev);
            vector<TreeNode *> cur;
            for(auto i = prev.begin(); i != prev.end(); i++){
                if((*i)->left) cur.push_back((*i)->left);
                if((*i)->right) cur.push_back((*i)->right);
            }
            prev = cur;
        }
        
        for(auto i = levels.rbegin(); i != levels.rend(); i++){
            vector<int> tmp;
            for(auto j = i->begin(); j != i->end(); j++)
                tmp.push_back((*j)->val);
            result.push_back(tmp);
        }
        
        return result;
    }
};