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：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        vector<int> res;
        queue<TreeNode*> q;
        if(root) q.push(root);
        int tmpmax;
        while(!q.empty()) {
            int sz = q.size();
            tmpmax = q.front()->val;
            for(int i=0; i<sz; i++) {
                TreeNode* cur = q.front();
                q.pop();
                tmpmax = max(tmpmax, cur->val);
                if(cur->left) q.push(cur->left);
                if(cur->right) q.push(cur->right);
            }
            res.push_back(tmpmax);
        }
        return res;
    }
};