1 BFS 双端队列
代码语言：javascript
复制/**
* 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) {
if (!root) return {};
queue<TreeNode*> q;
q.push(root);
bool leftflag = true; // 从左至右标致
vector<vector<int>> res;
while (!q.empty()) {
deque<int> lst; // 双端队列，每层结束后需重置
int sz = q.size();
for (int i = 0; i < sz; i ) {
auto node = q.front(); q.pop();
if (leftflag) lst.push_back(node->val); // 若是从左至右，则顺序添加到双端队列中
else lst.push_front(node->val); // 若是从右至左，则逆序添加到双端队列中
if (node->left) q.push(node->left);
if (node->right) q.push(node->right);
}
res.push_back(vector<int>{lst.begin(), lst.end()}); // 最后将当前双端队列，顺序添加到res中，注意vector<int>强转类型
leftflag = !leftflag;
}
return res;
}
};
