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It is my interview question, use merge sort!!! There are some property:
1. index of left is always less than right
2. since i < j, s.t. a[i] > a[j], so we just need to calculate when a[i] > a[j]
Error:
1. we need to first calculate local then global, since we use & for A
2. know what you are doing!!! You are calculating some index