题目描述
给定一个二叉树,返回其节点值自底向上的层次遍历。 (即按从叶子节点所在层到根节点所在的层,逐层从左向右遍历)
例如:
给定二叉树 [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其自底向上的层次遍历为:
[
[15,7],
[9,20],
[3]
]
算法
宽度优先遍历
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 == NULL) return{};
queue<TreeNode*> q;
vector<vector<int> > res;
q.push(root);
while (!q.empty())
{
int len = q.size();
vector<int> s;
for (int i = 0; i < len; ++i)
{
TreeNode* tmp = q.front();
s.push_back(tmp->val);
q.pop();
if (tmp->left) q.push(tmp->left);
if (tmp->right) q.push(tmp->right);
}
res.push_back(s);
}
reverse(res.begin(), res.end());
return res;
}
};