1. N 叉树的层序遍历
   

vector<vector<int>> levelOrder(Node* root) {vector<vector<int>> ret;if (root == nullptr) return ret;queue<Node*> q;q.push(root);ret.push_back({root->val});int size = 1;while (!q.empty()) {vector<int> v;while (size--) {Node* front = q.front();q.pop();for (Node* e : front->children) {if (e != nullptr) {q.push(e);v.push_back(e->val);}}}if (!v.empty()) {ret.push_back(v);}size = q.size();}return ret;
}

2. 二叉树的锯齿形层序遍历
   

vector<vector<int>> zigzagLevelOrder(TreeNode* root) {vector<vector<int>> ret;if (root == nullptr) return ret;queue<TreeNode*> q;q.push(root);int level = 1;while (!q.empty()) {vector<int> v;int size = q.size();while (size--) {TreeNode* front = q.front();q.pop();v.push_back(front->val);if (front->left != nullptr) {q.push(front->left);}if (front->right != nullptr) {q.push(front->right);}}if (level % 2 == 0) {reverse(v.begin(), v.end());}ret.push_back(v);++level;}return ret;
}

3. 二叉树最大宽度
   

int widthOfBinaryTree(TreeNode* root) {vector<pair<TreeNode*, unsigned int>> vp_prev = { {root, 1} };unsigned int max_size = 0;while (!vp_prev.empty()) {max_size = max(max_size, vp_prev.back().second - vp_prev.front().second + 1);vector<pair<TreeNode*, unsigned int>> vp;for (auto& [x, y] : vp_prev) {if (x->left != nullptr) {vp.push_back({ x->left, 2 * y });}if (x->right != nullptr) {vp.push_back({ x->right, 2 * y + 1 });}}vp_prev =  vp;}return max_size;
}

4. 在每个树行中找最大值
   

vector<int> largestValues(TreeNode* root) {vector<int> ret;if (root == nullptr) {return ret;}queue<TreeNode*> q;q.push(root);while (!q.empty()) {int max_val = INT_MIN;int size = q.size();while (size--) {TreeNode* front = q.front();q.pop();if (front->left != nullptr) {q.push(front->left);}if (front->right != nullptr) {q.push(front->right);}max_val = max(max_val, front->val);}ret.push_back(max_val);}return ret;
}