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/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void bfs(vector<vector<int>>& levels,
vector<const TreeNode*>& current_level,
vector<const TreeNode*>& next_level) {
if (current_level.empty()) {
return;
}
levels.push_back({});
vector<int>& level = levels.back();
level.reserve(current_level.size());
for (const TreeNode* node : current_level) {
level.push_back(node->val);
if (node->left) {
next_level.push_back(node->left);
}
if (node->right) {
next_level.push_back(node->right);
}
}
current_level.clear(); // New next level.
bfs(levels, next_level, current_level);
}
vector<vector<int>> levelOrder(TreeNode* root) {
if (!root) return {};
vector<vector<int>> levels;
vector<const TreeNode*> current_level({root});
vector<const TreeNode*> next_level;
bfs(levels, current_level, next_level);
return levels;
}
};
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