/**
 * 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<int> levelOrder(TreeNode* root) {
        vector<int> res;
        if (!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            TreeNode* cur = q.front();
            res.push_back(cur->val);
            q.pop();
            if (cur->left != nullptr) q.push(cur->left);
            if (cur->right != nullptr) q.push(cur->right);
        }
        return res;
    }
};

/**
 * 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>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        vector<int> tmp;
        while (!q.empty()) {
            int s = q.size();
            tmp.clear();
            for (int i = 0; i < s; i++) {
                TreeNode* cur = q.front();
                tmp.push_back(cur->val);
                q.pop();
                if (cur->left != nullptr) q.push(cur->left);
                if (cur->right != nullptr) q.push(cur->right);
            }
            if (!tmp.empty()) res.push_back(tmp);
        }
        return res;
    }
};

/**
 * 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>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;

        queue<TreeNode*> q;
        q.push(root);
        q.push(nullptr);

        vector<int> level;
        bool zigzag = false;
        while (q.size()) {
            auto t = q.front();
            q.pop();
            if (!t) {
                if (level.empty()) break;
                if (zigzag) reverse(level.begin(), level.end());
                res.push_back(level);
                level.clear();
                q.push(nullptr);
                zigzag = !zigzag;
                continue;
            }
            level.push_back(t->val);
            if (t->left) q.push(t->left);
            if (t->right) q.push(t->right);
        }
        return res;
    }
};