kk-r

def is_valid_state(state):
    # check if it is a valid solution
    return True

def get_candidates(state):
    return []

def search(state, solutions):
    if is_valid_state(state):
        solutions.append(state.copy())

kk-r

# DFS
def preorder(self, root):
    if not root:
        return []
    ret = []
    stack = [root]
    while stack:
        node = stack.pop()
        ret.append(node.val)
        if node.right:

kk-r

# Iterative
def dfs(graph, start):
  visited, stack = set(), [start]
  while stack:
    node = stack.pop()
    visited.add(node)
    for neighbor in graph[node]:
      if not neighbor in visited:
        stack.append(neighbor)
  return visited