ZigZag Level Order Traversal BT

• Time : $O(n)$
• Space : $O(n)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void helper(vector<vector<int>>& res, int level, TreeNode* node) {
    if(!node) return;
    if(res.size() == level) res.emplace_back();
    res[level].push_back(node->val);
    helper(res,level + 1, node->left);
    helper(res,level + 1, node->right);
}
vector<vector<int>> Solution::zigzagLevelOrder(TreeNode* A) {
    vector<vector<int>> res;
    helper(res, 0, A);
    for(int i = 1; i < res.size(); i += 2) reverse(begin(res[i]), end(res[i]));
    return res;
}