LintCode-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）

您在真实的面试中是否遇到过这个题？

Yes

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]
]

标签 Expand

分析：就是层次遍历呀。。。

代码：

/**
* 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<TreeNode*> > ret;
if(root==nullptr)
return vector<vector<int> >();
vector<TreeNode*> cur;
cur.push_back(root);
ret.push_back(cur);
while(true)
{
vector<TreeNode*> newCur;
vector<TreeNode*> cur = ret.back();
for(auto x:cur)
{
if(x->left)
newCur.push_back(x->left);
if(x->right)
newCur.push_back(x->right);
}
if(newCur.size()>0)
ret.push_back(newCur);
else
break;
}
reverse(ret.begin(),ret.end());
vector<vector<int> > values;
for(auto v:ret)
{
vector<int> vals;
for(auto t:v)
vals.push_back(t->val);
values.push_back(vals);
}
return values;
}
};