算法

奇数行从左到又，偶数行从右到左
在层序遍历二叉树的基础上，判断当前层的奇偶性，奇数行正序输出，偶数行逆序输出

时间复杂度

参考文献

C++ 代码

/**
 * 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>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>>res;
        if(!root)return res;
        vector<int>level;
        vector<int>temp;
        queue<TreeNode*>q;
        q.push(root);
        q.push(nullptr);
        int n = 0;
        while(q.size()){
            auto t = q.front();
            q.pop();
            if(!t){
                n++;
                if(level.empty())break;
                if(n%2==0)reverse(level.begin(),level.end());
                res.push_back(level);
                level.clear();
                q.push(nullptr);
                continue;
            }
            level.push_back(t->val);
            if(t->left)q.push(t->left);
            if(t->right)q.push(t->right);

        }
        return res;

    }
};