- 将根节点插入队列中;
- 创建一个新队列,用来按顺序保存下一层的所有子节点;
- 对于当前队列中的所有节点,按顺序依次将儿子插入新队列;
- 按从左到右、从右到左的顺序交替保存队列中节点的值;
- 重复步骤2-4,直到队列为空为止。
/**
* 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>> res;
queue<TreeNode*> q;
q.push(root);
bool flag = false;
while (q.size()) {
int cnt = q.size();
vector<int> level(cnt);
for (int i = 0; i < cnt; i++) {
TreeNode* t = q.front();
q.pop();
level[i] = t->val;
if (t->left) q.push(t->left);
if (t->right) q.push(t->right);
}
if (flag) reverse(level.begin(), level.end());
res.push_back(level);
flag = !flag;
}
return res;
}
};