LeetCode 103. Binary Tree Zigzag Level Order Traversal

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class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        preorder(root, 0, ans);
        return ans;
    }
private:
    void preorder(TreeNode* root, int level, vector<vector<int>>& ans) {
        if (root == nullptr) return;
        if (level >= ans.size())
            ans.push_back(vector<int>());
        auto& row = ans[level];
        if (level % 2 == 0) row.push_back(root->val);
        else row.insert(row.begin(), root->val);
        preorder(root->left, level + 1, ans);
        preorder(root->right, level + 1, ans);
    }
};
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class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (root == nullptr) return ans;
        queue<TreeNode*> q;
        q.push(root);
        bool leftToRight = true;
        while (!q.empty()) {
            deque<int> row;
            int size = q.size();
            while (size--) {
                auto node = q.front(); q.pop();
                if (leftToRight) row.push_back(node->val);
                else row.push_front(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            ans.emplace_back(vector<int>{row.begin(), row.end()});
            leftToRight = !leftToRight;
        }
        return ans;
    }
};