Syntax error Program to check there is any forward path in circular cyclic list or not in Python

Program to check there is any forward path in circular cyclic list or not in Python

Suppose we have a circular list called nums. So the first and the last elements are neighbors. So starting from any index say i, we can move nums[i] number of steps forward if nums[i] is a positive value, otherwise backwards if it's a negative value. We have to check whether there is a cycle whose length is greater than one such that the path only goes forwards or only goes backwards.

So, if the input is like nums = [-1, 2, -1, 1, 2], then the output will be True, because there is a forward path [1 -> 3 -> 4 -> 1]

To solve this, we will follow these steps −

  • n := size of nums
  • if n is same as 0, then
    • return False
  • seen := an array of size n and fill with 0
  • update nums by getting x mod n for each element x in nums
  • iter := 0
  • for i in range 0 to n - 1, do
    • if nums[i] is same as 0, then
      • go for next iteration
    • iter := iter + 1
    • pos := True
    • neg := True
    • curr := i
    • Do the following repeatedly, do
      • if nums[curr] and seen[curr] is same as iter, then
        • return True
      • if seen[curr] is non-zero, then
        • come out from loop
      • if nums[curr] > 0, then
        • neg := False
      • otherwise,
        • pos := False
      • if neg and pos both are false, then
        • come out from the loop
      • seen[curr] := iter
      • curr := (curr + nums[curr] + n) mod n
      • if nums[curr] is same as 0, then
        • come out from the loop
  • return False

Example

Let us see the following implementation to get better understanding −

def solve(nums):
   n = len(nums)
   if n == 0:
      return False
   seen = [0]*n
   nums = [x % n for x in nums]
   iter = 0
   for i in range(n):
      if nums[i] == 0:
         continue
      iter += 1
      pos = True
      neg = True
      curr = i
      while True:
         if nums[curr] and seen[curr] == iter:
            return True
         if seen[curr] :
            break
         if nums[curr] > 0:
            neg = False
         else:
            pos = False
         if not neg and not pos:
            break
         seen[curr] = iter
         curr = (curr + nums[curr] + n) % n
         if nums[curr] == 0:
            break
   return False

nums = [-1, 2, -1, 1, 2]
print(solve(nums))

Input

[-1, 2, -1, 1, 2]

Output

True
Updated on: 2021-10-16T10:34:29+05:30

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