103. Binary Tree Zigzag Level Order Traversal

BFS O(n)

/**
 * 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;
        queue<TreeNode *> q{{root}};
        while (!q.empty()) {
            res.push_back({});
            for (int i = q.size(); i > 0; --i) {
                auto x = q.front(); q.pop();
                if (x) {
                    res.back().push_back(x->val);
                    q.push(x->left);
                    q.push(x->right);
                }
            }
            if (!(res.size() & 1)) {
                reverse(begin(res.back()), end(res.back()));
            }
        }
        res.pop_back();
        return res;
    }
};