LintCode-70.二叉树的层次遍历 II

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},

按照从下往上的层次遍历为：
[
    [15,7],
    [9,20],
    [3]
]

标签

二叉树 队列 二叉树遍历 宽度优先搜索

code

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root : The root of binary tree.
     * @return : buttom-up level order a list of lists of integer
     */
public:
    vector<vector<int>> levelOrderBottom(TreeNode *root) {
        // write your code here
        
        vector<vector<int> > order;
        queue<TreeNode*> queue;
        int len;

        if(root == NULL) {
            return order;
        }

        queue.push(root);
        len = queue.size();

        while(!queue.empty())  {  
            vector<int> base;  
            len = queue.size();  
  
            while(len--) {
                TreeNode *tmp=queue.front();  
                base.push_back(tmp->val);
                queue.pop();  
                if(tmp->left)  
                    queue.push(tmp->left);  
                if(tmp->right)      
                    queue.push(tmp->right);  
            }  
            order.push_back(base);  
        }  

        vector<vector<int> > order2;
        int i;
    
        for(i=order.size()-1; i>=0; i--) {
            order2.push_back(order[i]);
        }

        return order2;
    }
};