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] ]
Related Topics:
Tree, Breadth-first Search
Similar Questions:
// OJ: https://leetcode.com/problems/binary-tree-level-order-traversal-ii/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
if (!root) return {};
vector<vector<int>> ans;
queue<TreeNode*> q;
q.push(root);
while (q.size()) {
int cnt = q.size();
ans.emplace_back();
while (cnt--) {
root = q.front();
q.pop();
ans.back().push_back(root->val);
if (root->left) q.push(root->left);
if (root->right) q.push(root->right);
}
}
reverse(begin(ans), end(ans));
return ans;
}
};