Isomorphic strings in LeetCode
I don't know how much runtime you'll save, but you're iterating 175 more times  than you need to here. 


int[] map = Enumerable.Repeat(-1, 175).ToArray();
bool[] marked = Enumerable.Repeat(false, 175).ToArray();



You can replace this with an old fashioned for loop and set them both at once. 

const int maxChars = 175;

int[] map = new int[maxChars];
bool[] marked = new bool[maxChars];

for (var i = 0; i &lt; maxChars; i++)
{
    map[i] = -1; 
    marked[i] = false;
}


It's more verbose, but might save you a millisecond or 2.