LeetCode: Deepest Leaves Sum C#
Rename DFS to DeepestLeavesSum.

Extract  depth + 1 to variable. 

In here else if (depth + 1 == maxDepth) the else is reduant.

The order of depth + 1 and maxDepth is different between the Ifs. One time  maxDepth is on the left side and other time on the right side.

Usually, when writing recursion, you return the value and not store it in the class. 

If you call DeepestLeavesSum twice with the same object the second call will return wrong results because it will use sum and maxDepth of the first call. A good practice is avoiding saving state.