IntPerm

Permutation -> integer conversion. (Not order preserving.) Integer -> permutation can use a lookup table.

Time: O(n) 6 lines

content/combinatorial/IntPerm.h — Simon Lindholm

int permToInt(vi& v) {
	int use = 0, i = 0, r = 0;
	for(int x:v) r = r * ++i + __builtin_popcount(use & -(1<<x)),
		use |= 1 << x;                     // (note: minus, not ~!)
	return r;
}