Let write some permutations of positive integers.
1: 1 2 3 4 5 6 ...
2: 2 1 3 4 ...
3: 1 3 2 4 ...
4: 2 3 1 4 ...
5: 3 1 2 4 ...
6: 3 2 1 4 ...
25: 1 2 3 5 4 ...
120: 5 4 3 2 1 6 7 ...Given a permutation of integers in range 1:n, return it's index following above pattern.
See also: 42495
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