// Time: O(n) where n is number of bits
// Space: O(1)
public static int GetPrevious(int n)
{
int temp = n;
int trailingOnes = 0;
int trailingZerosAfterOnes = 0;
while ((temp & 1) == 1)
{
trailingOnes++;
temp >>= 1;
}
while ((temp & 1) == 0 && temp != 0)
{
trailingZerosAfterOnes++;
temp >>= 1;
}
if (trailingZerosAfterOnes == 0 ||
trailingZerosAfterOnes + trailingOnes == 31)
{
return -1;
}
int rightMostNonTrailingOne = trailingZerosAfterOnes + trailingOnes;
// Clear right most non trailing 1.
n = BinaryHelpers.ClearBit(n, rightMostNonTrailingOne);
// Set all ones below cleared bit.
n |= (1 << rightMostNonTrailingOne) - 1;
// Add back zeros in least significat bits.
n &= -1 << (trailingZerosAfterOnes -1);
return n;
}