二叉树的之字形层序遍历

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > zigzagLevelOrder(TreeNode* root) {
        // 和按层次遍历一样，只是这道题遇到偶数行需要翻转，使用一个标记位标记一下就可以了
        vector<vector<int> > ans;
        if (root == nullptr)
            return ans;
        queue<TreeNode*> q;
        q.emplace(root);
        bool flag = true;
        while(!q.empty()) {
            int sz = q.size();
            vector<int> level;

            for (int i = 0; i < sz; i++) {
                TreeNode* node = q.front();
                q.pop();
                level.emplace_back(node->val);
                if (node->left)
                    q.emplace(node->left);
                if (node->right)
                    q.emplace(node->right);
            }
            if (!flag) {
                reverse(level.begin(), level.end());
            }
            ans.emplace_back(level);
            flag = !flag;

        }

        return ans;
    }
};