LeetCode 515 在每个数行中找最大值[BFS 二叉树] HERODING的LeetCode之路

解题思路：
BFS遍历每一层，统计每一层的最大值放入数组中，代码如下：

/**
 * 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) {
        if(root == nullptr) return {};
        queue<TreeNode*> q;
        q.emplace(root);
        vector<int> ans;
        while(!q.empty()) {
            int n = q.size();
            int maxNum = INT_MIN;
            for(int i = 0; i < n; i ++) {
                TreeNode* node = q.front();
                q.pop();
                maxNum = max(maxNum, node->val);
                if(node->left != nullptr) q.emplace(node->left);
                if(node->right != nullptr) q.emplace(node->right);
            }
            ans.emplace_back(maxNum);
        }
        return ans;
    }
};