0103. Binary Tree Zigzag Level Order Traversal.md

103. Binary Tree Zigzag Level Order Traversal, medium

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example: Given binary tree [3,9,20,null,null,15,7], 3 /
9 20 /
15 7 return its zigzag level order traversal as: [ [3], [20,9], [15,7] ]

// 8ms, 91%
/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;
        
        vector<TreeNode*> q1(1, root);
        bool reverse = false;
        while (!q1.empty()) {
            vector<TreeNode*> q2;
            res.push_back({});
            res.back().reserve(q1.size()*2);
            for (auto t : q1) {
                if (t->left) q2.push_back(t->left);
                if (t->right) q2.push_back(t->right);

                if (!reverse) {
                    res.back().push_back(t->val);
                }
            }
            if (reverse) {
                for (int i = q1.size()-1; i >= 0; i--) {
                    res.back().push_back(q1[i]->val);
                }
            }
            reverse = !reverse;
            q1.swap(q2);
        }
        return res;
    }
};