Function maxPathSum

kotlin/Trees/MaximumPathSum.kt:13–19

*/

fun maxPathSum(root: TreeNode?): Int {
    // In Kotlin, global variable is bad practice.
    // So, we use a mutable list to store the max sum and access it by reference.
    var maxSum = mutableListOf(Int.MIN_VALUE)
    maxPathSumHelper(root, maxSum)
    return maxSum[0]
}

fun maxPathSumHelper(node: TreeNode?, maxSum: MutableList<Int>): Int {
    // Base case: null nodes have no path sum.