103.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) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> res;
        queue<TreeNode*> q;
        if (root) q.push(root);
        bool left2right = true;
        while (!q.empty()) {
            int n = q.size();
            vector<int> collection;
            while (n--) {
                TreeNode* node = q.front();
                q.pop();
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
                collection.push_back(node->val);
            }
            if (!left2right) {
                reverse(collection.begin(), collection.end());
            }
            res.push_back(collection);
            left2right ^= 1;
        }
        return res;
    }
};