How can I return a sorted list of integers here? [closed]

def pairs (x):
    i = 0  
    z = []  
    while i < len(x)-1:
       if x[i] == x[i+1]:
            z.append(x[i])
            i+=1
       i+=1
    return z

print (pairs([4, 5, 6, 6, 6, 6, 9, 12, 12]))
print (pairs([4, 5, 4, 10, 10, 8, 9, 8,9]))

My problem is on the last line. In the last line, I wanted to return 4, 8, 9, 10 but it’s only returning 10 instead. Where should I put the sort operation? I tried assigning first, it worked, but only on the assigned value and not whatever the test case would use.

Answer

I used a set to track pair elements and sorted the resulting list.

Program:

def pairs(x) :
    i = 0
    nums = set()
    pairs = []
    for item in x:
        if item in nums:
            pairs.append(item)
            nums.remove(item)
        else:
            nums.add(item)
    pairs.sort()
    return pairs
        

print (pairs([4, 5, 4, 10, 10, 8, 9, 8,9]))

Result:

[4, 8, 9, 10]