use array to replace HashMap when then key is single character

int[] counts = new int[128]
foreach(var c in "hello, world！"）
    counts[c]++;

use x%2!=0 to determine if x is odd, instead of x%2==1, because of -1%2=-1

string format

This is just a C# string format tips

// In the format {0,-20} means the first argument has a fixed length 20, 
//    and the negative sign guarantees the string is printed from left to right.
string format = "{0,-20} {1,-10}";