LeetCode/cs/kth-largest-sum-in-a-binary-tree.cs

public class Solution {
    private void Sum(List<long> levels, TreeNode node, int level) {
        if (node == null) {
            return;
        }

        while (level >= levels.Count) {
            levels.Add(0);
        }

        levels[level] += node.val;
        Sum(levels, node.left, level + 1);
        Sum(levels, node.right, level + 1);
    }

    public long KthLargestLevelSum(TreeNode root, int k) {
        var levels = new List<long>();

        Sum(levels, root, 0);
        if (k > levels.Count) {
            return -1;
        }

        levels.Sort();
        return levels[levels.Count - k];
    }
}