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#!/usr/bin/env python3
from collections import Counter
def merge_items(items1, items2):
''' Given two 2-D array of positive integers, items1 and items2 where
element is pair of (item_id, item_quantity), return the merged items.
>>> merge_items([ [1,1], [2,1], [3,2] ], [ [2,2], [1,3] ])
[[1, 4], [2, 3], [3, 2]]
>>> merge_items([[1, 2], [2, 3], [1, 3], [3, 2]], [[3, 1], [1, 3]])
[[1, 8], [2, 3], [3, 3]]
>>> merge_items([[1, 1], [2, 2], [3, 3]], [[2, 3], [2, 4]])
[[1, 1], [2, 9], [3, 3]]
'''
counter = Counter()
all_items = items1 + items2
for item in all_items:
counter[item[0]] += item[1]
return [[item_id, counter[item_id]] for item_id in sorted(counter.keys())]
if __name__ == "__main__":
import doctest
doctest.testmod()
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