面试题32 - I. 从上到下打印二叉树
从上到下打印出二叉树的每个节点,同一层的节点按照从左到右的顺序打印。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回:
[3,9,20,15,7]
提示:
节点总数 <= 1000
思路:迭代法–BFS
/**
* 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<int> levelOrder(TreeNode* root) {
vector<int> ans;
if (root == NULL){
return ans;
}
queue<TreeNode*> q;
q.push(root);
TreeNode* pNode = NULL;
while (!q.empty()){
pNode = q.front();
q.pop();
ans.push_back(pNode->val);
if (pNode->left){
q.push(pNode->left);
}
if(pNode->right){
q.push(pNode->right);
}
}
return ans;
}
};
/*12ms,14.7MB*/
时间复杂度:O(n)
空间复杂度:O(n)