Syntax error Program to find number of minimum steps to reach last index in Python

Program to find number of minimum steps to reach last index in Python

Suppose we have a list of numbers called nums and we are placed currently at nums[0]. On each step, we can either jump from the current index i to i + 1 or i - 1 or j where nums[i] == nums[j]. We have to find the minimum number of steps required to reach the final index.

So, if the input is like nums = [4, 8, 8, 5, 4, 6, 5], then the output will be 3, as we can jump from index 0 to index 4 as their values are both 4. And then we jump back to index 3. Finally, we can jump from index 3 to 6 since both of their values are 5.

To solve this, we will follow these steps −

  • pos := an empty map
  • for each index i, and value n in nums, do
    • insert i at the end of pos[n]
  • n := size of nums
  • visited := make a list of size n, and fill this with False
  • visited[0] := True
  • while q is not empty, do
    • (u, d) := left element of q, and delete left element
    • if u is same as n - 1, then
      • return d
    • for each v in the lists pos[nums[u]] and [u - 1, u + 1], do
      • if 0 <= v < n and visited[v] is false, then
        • visited[v] := True
        • insert pair (v, d + 1) at the end of q
    • remove pos[nums[u]]

Let us see the following implementation to get better understanding −

Example

 Live Demo

class Solution:
   def solve(self, nums):
      from collections import defaultdict, deque
      pos = defaultdict(list)
      for i, n in enumerate(nums):
         pos[n].append(i)
      q = deque([(0, 0)])
      n = len(nums)
      visited = [False] * n
      visited[0] = True
      while q:
         u, d = q.popleft()
         if u == n - 1:
            return d
         for v in pos[nums[u]] + [u - 1, u + 1]:
            if 0 <= v < n and not visited[v]:
               visited[v] = True
               q.append((v, d + 1))
         del pos[nums[u]]
ob = Solution()
nums = [4, 8, 8, 5, 4, 6, 5]
print(ob.solve(nums))

Input

[4, 8, 8, 5, 4, 6, 5]

Output

3
Updated on: 2020-11-19T06:19:24+05:30

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