<p>Given the <code>root</code> of a binary tree, the level of its root is <code>1</code>, the level of its children is <code>2</code>, and so on.</p>

<p>Return the <strong>smallest</strong> level <code>x</code> such that the sum of all the values of nodes at level <code>x</code> is <strong>maximal</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/05/03/capture.JPG" style="width: 200px; height: 175px;" />
<pre>
<strong>Input:</strong> root = [1,7,0,7,-8,null,null]
<strong>Output:</strong> 2
<strong>Explanation: </strong>
Level 1 sum = 1.
Level 2 sum = 7 + 0 = 7.
Level 3 sum = 7 + -8 = -1.
So we return the level with the maximum sum which is level 2.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [989,null,10250,98693,-89388,null,null,null,-32127]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>-10<sup>5</sup> &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
</ul>

class Solution {
public:
    int Height(TreeNode* root) {
        if (!root) return 0;
        return std::max(Height(root->left), Height(root->right)) + 1;
    }

    void collectLevel(TreeNode* root, int level, std::vector<int>& vec) {
        if (!root) return;
        if (level == 1) {
            vec.push_back(root->val);
        } else {
            collectLevel(root->left, level - 1, vec);
            collectLevel(root->right, level - 1, vec);
        }
    }

    int maxLevelSum(TreeNode* root) {
        int height = Height(root);
        int maxSum = INT_MIN;
        int maxLevel = -1;
        
        for (int i = 1; i <= height; ++i) {
            std::vector<int> vec;
            collectLevel(root, i, vec);
            int currSum = std::accumulate(vec.begin(), vec.end(), 0);
            if (maxSum < currSum) {
                maxSum = currSum;
                maxLevel = i;
            }
        }
        return maxLevel;
    }
};

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