Binary Tree Zigzag Level Order Traversal.cpp

/**
 * 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) {}
 * };
 */

#include <vector>

using namespace std;


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:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root)
    {
        vector<vector<int>> res;
        helper(root, 0, res);
        return res;
    }

    void helper(TreeNode *node, int level, vector<vector<int>> &res)
    {
        if(node == nullptr) return;
        if(res.size() <= level)
        {
            res.push_back({});
        }
        vector<int> &oneLevel = res[level];
        if(level % 2 == 0) oneLevel.push_back(node->val);
        else oneLevel.insert(oneLevel.begin(), node->val);
        helper(node->left, level + 1, res);
        helper(node->right, level + 1, res);
    }
};