[515] 在每个树行中找最大值

您需要在二叉树的每一行中找到最大的值。
示例：

输入: 
          1
         / \
        3   2
       / \   \  
      5   3   9 
输出: [1, 3, 9]

/**
 * 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;
        if(root == nullptr) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            int levelSize = q.size();
            int max_val = q.front()->val;
            for(int i=0;i<levelSize;i++){
                TreeNode* cur = q.front();
                max_val = max(max_val, cur->val);
                q.pop();
                if(cur->left){
                    q.push(cur->left);
                } 
                if(cur->right) {
                    q.push(cur->right);
                }
            }  
            res.push_back(max_val);        
        }
        return res;

    }
};