题目
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,#,#,15,7},
3
/ \
9 20
/ \
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
解题之法
/**
* 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) {
vector<vector<int> > res;
levelorder(root, 0, res);
return vector<vector<int> > (res.rbegin(), res.rend());
}
void levelorder(TreeNode *root, int level, vector<vector<int> > &res) {
if (!root) return;
if (res.size() == level) res.push_back({});
res[level].push_back(root->val);
if (root->left) levelorder(root->left, level + 1, res);
if (root->right) levelorder(root->right, level + 1, res);
}
};
