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]
]
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这个就是二叉树的层序遍历，和leetcode102. Binary Tree Level Order Traversal类似，只是要把最后得到的二维数组逆转一下就成。
C++代码：
/**
* 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) return {};
queue<TreeNode*> q;
q.push(root);
vector<vector<int> > vec;
while(!q.empty()){
vector<int> v;;
for(int i = q.size(); i > ; i--){
auto t = q.front();
q.pop();
v.push_back(t->val);
if(t->left) q.push(t->left);
if(t->right) q.push(t->right);
}
vec.push_back(v);
}
reverse(vec.begin(),vec.end());
return vec;
}
};