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What you have got is called post inflammatory hyper pigmentation.Treatment depends upon the depth of pigmentation.If it is superficial then, chemical peels
creams ,tretinon creams might help you.
But if it is deep .then lasers are best options.
The specific laser would be Q-switched nd:yag laser which works good on pigmentation .
kindly consult a dermatologist
who has a good laser practice also and can help you with the same.