Given an array, count number of pairs that can be formed from all possible contiguous sub-arrays containing distinct numbers. The array contains positive numbers between 0 to n-1 where n is the size of the array.
Input: [1, 4, 2, 4, 3, 2] Output: 8 The subarrays with distinct elements are [1, 4, 2], [2, 4, 3] and [4, 3, 2]. There are 8 pairs that can be formed from above array. (1, 4), (1, 2), (4, 2), (2, 4), (2, 3), (4, 3), (4, 2), (3, 2) Input: [1, 2, 2, 3] Output: 2 There are 2 pairs that can be formed from above array. (1, 2), (2, 3)
The idea is to use Sliding Window for the given array. Let us use a window covering from index left to index right and an Boolean array visited to mark elements in current window. The window invariant is that all elements inside the window are distinct. We keep on expanding the window to the right and if a duplicate is found, we shrink the window from left till all elements are distinct again. We update the count of pairs in current window along the way. An observation showed that in an expanding window, number of pairs can be incremented by value equal to window size – 1. For example,
Expanding Window Count  Count = 0 [1, 2] Count += 1 pair i.e. (1, 2) [1, 2, 3] Count += 2 pairs i.e. (1, 3) and (2, 3) [1, 2, 3, 4] Count += 3 pairs i.e. (1, 4), (2, 4) and (3, 4)
So, total Count for above window of size 4 containing distinct elements is 6. Nothing need to be done when the window is shrinking.
Below is the implementation of the idea.
Time Complexity: The complexity might look O(n^2) as 2 while loop are involved but note that left and right index are changing from 0 to N-1. So overall complexity is O(n + n) = O(n). Auxiliary space required in above solution is O(n) as we are using visited array to mark elements of the current window.
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