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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