【一次过】Lintcode 70. 二叉树的层次遍历 II

给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）

样例

给出一棵二叉树 {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

按照从下往上的层次遍历为：

[
  [15,7],
  [9,20],
  [3]
]

解题思路：

    与Lintcode 69:Binary Tree Level Order Traversal思路一致，只是逆序输出，所以用stack暂存，最后取出即可。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: A tree
     * @return: buttom-up level order a list of lists of integer
     */
    vector<vector<int>> levelOrderBottom(TreeNode * root) 
    {
        // write your code here
        vector<vector<int>> res;
        stack<vector<int>> stk;
        if(root == NULL)
            return res;
            
        queue<TreeNode *> q;
        q.push(root);
        
        while(!q.empty())
        {
            vector<int> tmp;
            int len = q.size();
            
            while(len--)
            {
                TreeNode * node = q.front();
                q.pop();
                
                tmp.push_back(node->val);
                
                if(node->left)
                    q.push(node->left);
                if(node->right)
                    q.push(node->right);
            }
            
            stk.push(tmp);
        }
        
        while(!stk.empty())
        {
            res.push_back(stk.top());
            stk.pop();
        }
        
        return res;
    }
};