dictionary = {i:chr(i) for i in range(97,123)}
last = 256
arr = [97, 97, 98, 256, 258, 257, 259]

result = []
p = arr.pop(0)
result.append(dictionary[p])

for c in arr:
	if c in dictionary:
		entry = dictionary[c]
	result.append(entry)
	dictionary[last] = dictionary[p] + entry[0]
	last += 1
	p = c
	
print(''.join(result))

Lempel Ziv Welch Decompression in python

LZW Decompression

Python