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;
}