C :: Segmentation Fault When Replacing A Char In Array

This function should replace all instances of a character in a given character array, while returning the amount of characters changed, but I keep getting a segmentation fault at the highlighted area.

I'm only supposed to use pointers so arrays are out of the question, and I don't think we are allowed to use the string.h library as well. How I could avoid something the segmentation fault or ways to fix it?

Code:

int replaceChars(char replace, char find, char *input) { int i, j;
//Finds length
for(i = 0; *(input + (i + 1)) != ''; i++);
int c = 0;
for(j = 0; j <= i; j++) {
if(*(input + j) == find) {
*(input + j) = replace; //Segmentation fault happens here
c++;
} }
return c;
}