515. Find Largest Value in Each Tree Row

O(n) BFS

/**
 * 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) return {};
        vector<int> res;
        queue<TreeNode *> q{{root}};
        while (!q.empty()) {
            int v = INT_MIN;
            for (int i = q.size(); i > 0; --i) {
                auto x = q.front(); q.pop();
                v = max(v, x->val);
                if (x->left) {
                    q.push(x->left);
                }
                if (x->right) {
                    q.push(x->right);
                }
            }
            res.push_back(v);
        }
        return res;
    }
};