Tree

self.maxDistance = 0
def dfs(node):
    depth = 0
    if not node:
        return depth
    leftDepth = dfs(node.left)
    rightDepth = dfs(node.right)
    if node.val == start:
        self.maxDistance = max(leftDepth, rightDepth)
        depth = -1
    elif leftDepth >= 0 and rightDepth >= 0:
        depth = max(leftDepth, rightDepth) + 1
    else:
        distance = abs(leftDepth) + abs(rightDepth)
        self.maxDistance = max(self.maxDistance, distance)
        depth = min(leftDepth, rightDepth) - 1
    return depth
dfs(root)
return self.maxDistance