515. Find Largest Value in Each Tree Row

问题：

给定二叉树，求每层最大节点，返回。

Example 1:
Input: root = [1,3,2,5,3,null,9]
Output: [1,3,9]

Example 2:
Input: root = [1,2,3]
Output: [1,3]

Example 3:
Input: root = [1]
Output: [1]

Example 4:
Input: root = [1,null,2]
Output: [1,2]

Example 5:
Input: root = []
Output: []
 
Constraints:
The number of nodes in the tree will be in the range [0, 104].
-231 <= Node.val <= 231 - 1

example 1:

解法：BFS

queue存储每层node，处理每层节点时，对每个节点求max。

代码参考：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 8  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 9  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
10  * };
11  */
12 class Solution {
13 public:
14     vector<int> largestValues(TreeNode* root) {
15         vector<int> res;
16         queue<TreeNode*> q;
17         if(root) q.push(root);
18         int tmpmax;
19         while(!q.empty()) {
20             int sz = q.size();
21             tmpmax = q.front()->val;
22             for(int i=0; i<sz; i++) {
23                 TreeNode* cur = q.front();
24                 q.pop();
25                 tmpmax = max(tmpmax, cur->val);
26                 if(cur->left) q.push(cur->left);
27                 if(cur->right) q.push(cur->right);
28             }
29             res.push_back(tmpmax);
30         }
31         return res;
32     }
33 };