Struggling to figure out how to most efficiently solve a problem.
Given a list of lists, say [[4, 4], [4, 5, 6], [8, 9, 1]], and a second list, say [8, 9, 7, 1], how would you programmatically (Python) find the max number of lists from the first list that can be subtracted from the second list? The order of the lists must remain. So using the examples, the max would be 2 because:
[4, 4] goes into first and second elements of second list -> [4, 5, 7, 1]
[4, 5, 6] goes into one, second, and third elements of resulting list above -> [0, 0, 1, 1]
If we were to try and put [8, 9, 1] into [8, 9, 7, 1], we’d get [0, 0, 6, 1] remainder and neither [4, 4] nor [4, 5, 6] could fit into that.
I hope that makes sense! I’ve been struggling to word it.
Thanks
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I would use the
our problem can be approached using a backtracking algorithm. You may try each list from the first list, subtract it from the second list (if possible), and then recursively try to fit the next list from the first list into the remainder of the second list. The goal is to find the maximum number of lists that can be subtracted in this manner.
I would approach it like following:
def can_subtract(list1, list2):
"""
Try to subtract list1 from list2 starting from any position.
Returns the resulting list if possible, otherwise None.
"""
n, m = len(list1), len(list2)
for i in range(m - n + 1): # Try all starting positions in list2
if all(list2[i+j] >= list1[j] for j in range(n)):
# Subtract list1 from list2 starting from index i
new_list2 = list2[:]
for j in range(n):
new_list2[i+j] -= list1[j]
return new_list2
return None
def max_lists_subtracted(lists, target_list):
def backtrack(index, current_list, count):
nonlocal max_count
if index >= len(lists):
max_count = max(max_count, count)
return
# Try to subtract the current list
result = can_subtract(lists[index], current_list)
if result is not None:
# If successful, move to the next list
backtrack(index + 1, result, count + 1)
# Also consider not using the current list and moving to the next
backtrack(index + 1, current_list, count)
max_count = 0
backtrack(0, target_list, 0)
return max_count
# Example usage:
lists = [[4, 4], [4, 5, 6], [8, 9, 1]]
target_list = [8, 9, 7, 1]
print(max_lists_subtracted(lists, target_list)) # Output is 2
So, here
can_subtract(list1, list2)
: Checks iflist1
can be subtracted fromlist2
starting at any position. If possible, it returns the modifiedlist2
after subtraction; otherwise, it returnsNone
.- Secondly,
max_lists_subtracted(lists, target_list)
uses backtracking to try subtracting each list in lists from thetarget_list
. It keeps track of the maximum number of lists that can be subtracted.