import hou
startNode = hou.node("/obj/geo1/null1")

nodeList = []
def findMerge(node):
    for input in node.inputs(): #Get a list of all node connected to the current node
        if "merge" == input.type().nameComponents()[2]: #We want to make sure we don't mistake an object merge for a merge by exactly comparing with the 'name' component of the node, ignoring version or namespace.
            findMerge(input) #if the node if a merge, we call the function.
        else:
            nodeList.append(input) #if it not a merge node, we add it do the list

findMerge(startNode) #Call the function with the initial node
print(nodeList)