算法1

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) {
        if(!root) return {};
        vector<vector<int>> ans;
        deque<TreeNode*> q;
        q.push_back(root);
        int cnt = 1;

        while(!q.empty()){
            int len = q.size();
            vector<int> f;
            while(len --){
                if(cnt % 2 == 1){
                    auto cur = q.front(); q.pop_front();
                    f.push_back(cur->val);
                    if(cur->left) q.push_back(cur->left);
                    if(cur->right) q.push_back(cur->right);
                } else{
                    auto cur = q.back(); q.pop_back();
                    f.push_back(cur->val);
                    if(cur->right) q.push_front(cur->right);
                    if(cur->left) q.push_front(cur->left);
                }
            }
            cnt ++;
            ans.push_back(f);
        }
        return ans;
    }
};