103. Binary Tree Zigzag Level Order Traversal

/**
 * 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) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if(root == NULL)
            return ans;

        queue<TreeNode*> q;
        bool leftToRight = true;

        q.push(root);

        while(!q.empty())
        {
            int size = q.size();
            vector<int> temp(size);

            //accessing the elements of each level
            for(int i=0; i<size; i++)
            {
                TreeNode* front = q.front();
                q.pop();

                int index = leftToRight ? i : size - i - 1;

                temp[index] = front -> val;

                if(front -> left)
                {
                    q.push(front -> left);
                }

                if(front -> right)
                {
                    q.push(front -> right);
                }
            }

            //to change the direction
            leftToRight = !leftToRight;
            ans.push_back(temp);
        }

        return ans;
    }
};