Frequency→Triangular number

Q: Given a sequence A, find the number of i, j satisfying $A_i=A_j(i<j)$. Find the number of i, j satisfying [$ A_i = A_j (i < j)

• Constraints |A| = 10^7, 0 < Ai < 100

• Create a frequency table and find it by trigonometric number. Linear order.

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