I encountered a sentence in a very sad story by Ba Jin:
十二年，多么长的日日夜夜！每次我回到家门口，眼前就出现一张笑脸，一个亲切的声音向我迎来，可是走进院子，却只见一些高高矮矮的没有花的绿树。
My question is, what 高高矮矮 means. It is the reduplication of 高矮, which usually appears to be a noun for „height“. Since here it operates as an adjective, I assume it means „tall or short“, possibly with the connotation that the author does not care about their height.
It seems the author returned home after a long period of absence. He felt heartbreak and lost because the person who used to greet his homecoming with heartwarming smiles was gone, but the yard of unattended, wildly grown, green trees.
高高矮矮 means "uneven in height" - some are tall and some are short. Used here, it describes the scene of unattended trees that grows wild, as the person who used to tending the garden/yard has left or passed away.
When you see an AABB type word with A and B are antonyms, generally the author is describing both A and B.
So it's actually "tall AND short".
高高矮矮的树 = both tall trees and small trees = Some of the trees are tall and others are small
In this case it is just an imagery, the author does not tend to describe the height of the trees, but only to make the scene more vivid.
More examples:
日日夜夜：days and nights (in your sentence)
里里外外：inside and outside
是是非非：right and wrong
前前后后：before and after
真真假假：true and false
高矮 = height (n)
長短 = length (n)
大小 = size (n)
高高矮矮的 = (有些)高的，(有些)矮的 = some are tall, some are short = both tall and short (of different height)
長長短短的 = (有些)長的，(有些)短的 = some are long, some are short = both long and short (of different length)
大大小小的 = (有些)大的，(有些)小的 = some are big, some are small = both big and small (of different size)
高高矮矮的没有花的绿树 = flowerless green tree of different height
It is the reduplication of 高矮
高高矮矮的 is a reduplication of 高的 (adj 1) + 矮的 (adj 2). The reduplication indicates 'some (adj 1)' some (adj 2)
It is different from 快快樂樂地, which is a reduplication of 快樂地 (adverb). The reduplication emphasizes the adverb itself
Notice: 高高(的) is a reduplication of 高(的), it is for emphasis
The trees,some are tall, some are short (relatively).
