从上到下按层打印二叉树,同一层的节点按从左到右的顺序打印,每一层打印到一行。
例如:
给定二叉树: [3,9,20,null,null,15,7]
,
3
/ \
9 20
/ \
15 7
返回其层次遍历结果:
[
[3],
[9,20],
[15,7]
]
提示:
节点总数 <= 1000
来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof
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分析:
队列,广度优先搜索
对每层队列长度的循环
解答:
/**
* 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>> levelOrder(TreeNode* root) {
vector<vector<int>> ans;
if(root==NULL) return {};
queue<TreeNode*> que;
que.push(root);
while(!que.empty()){
vector<int> subans;
int subqlen = que.size();
for(int i=0;i<subqlen;i++){
TreeNode* tmpnode = que.front();
que.pop();
subans.push_back(tmpnode->val);
if(tmpnode->left) que.push(tmpnode->left);
if(tmpnode->right) que.push(tmpnode->right);
}
ans.push_back(subans);
}
return ans;
}
};