Program to find length of longest alternating subsequence from a given list in Python

Suppose we have a list of numbers called nums, we have to find the size of the longest subsequence where the difference between two consecutive numbers alternate between positive and negative. And the first difference may be either positive or negative.

So, if the input is like nums = [6, 10, 4, 2, 3, 9, 4, 7], then the output will be 6, as on possible required subsequence is [6, 10, 2, 9, 4, 7] and the differences are [4, -8, 7, -5, 3].

To solve this, we will follow these steps &minuS;

• n := size of nums
• dp := a list of size 2n and fill with 1
• ans := 0
• for i in range 0 to n, do
  • for j in range 0 to i, do
    • if nums[j] < nums[i], then
      • dp[i, 0] = maximum of dp[i, 0] and (dp[j, 1] + 1)
    • otherwise when nums[j] > nums[i], then
      • dp[i, 1] = maximum of dp[i, 1] and (dp[j, 0] + 1)
  • ans = maximum of ans, dp[i, 0] and dp[i, 1])
• return ans

Example (Python)

Let us see the following implementation to get better understanding −

 Live Demo

class Solution:
   def solve(self, nums):
      n = len(nums)
      dp = [[1] * 2 for _ in range(n)]
      ans = 0
      for i in range(n):
         for j in range(i):
            if nums[j] < nums[i]:
               dp[i][0] = max(dp[i][0], dp[j][1] + 1)
            elif nums[j] > nums[i]:
               dp[i][1] = max(dp[i][1], dp[j][0] + 1)
         ans = max(ans, dp[i][0], dp[i][1])
      return ans
ob = Solution()
nums = [6, 10, 4, 2, 3, 9, 4, 7]
print(ob.solve(nums))

Input

[6, 10, 4, 2, 3, 9, 4, 7]

Output

6