alternate char match in string - c

Question

Alternate code character should be same can you tell me how to correct my program as one test case shown by me wanted to executed exactly like that?
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
int main() {
int T, i;
char ticketnumber[102];
scanf("%d", &T);
while (T--) {
bool flag = false;
scanf("%s", ticketnumber);
for (i = 0; i < strlen(ticketnumber); i++) {
if ((ticketnumber[i] == ticketnumber[i + 2])
&& (ticketnumber[i] != ticketnumber[i + 1])) {
flag = true;
} else {
break;
}
}
if (flag != false) {
printf("YES");
} else {
printf("NO");
}
}
return 0;
}
Necessary: I/P:
3
CFCFCF
CRGHIT
CGIRST
O/P:
YES
NO
NO
Alternate code character should be same can you tell me how to correct my program?
If alternate char of string contains in whole length of string then we need to print "yes" else "No".

Answer 1

On the final iteration of the for loop, ticketnumber[i + 2] attempts to read the character positioned one past the null-terminating byte, potentially invoking Undefined Behaviour if this extends beyond the end of the array.
As is, flag is set to true upon completing a single iteration. This pattern will yield incorrect results for strings that fail the test after the first iteration, as flag is not set to false in this event.
With a problem like this, it is generally better to assume the data will pass the test (true), and end the procedure early when it does not (false).
As pointed out in the comments, instead of looking forward in the string, start iterating from the third position and look behind at the previous two positions.
There is a base case to consider: the empty string.
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
bool is_alternating(const char *s, size_t len)
{
if (0 == len)
return false; /* is an empty string alternating or not? */
if (2 == len)
return s[0] != s[1];
for (size_t i = 2; i < len; i++)
if (s[i] != s[i - 2] || s[i] == s[i - 1])
return false;
return true;
}
int main(void)
{
char buffer[512];
while (fgets(buffer, sizeof buffer, stdin)) {
size_t length = strcspn(buffer, "\r\n");
buffer[length] = 0;
printf("%s = %s\n", buffer,
is_alternating(buffer, length) ? "YES" : "NO");
}
}
Input:
CFCFCF
CRGHIT
CGIRST
Output:
CFCFCF = YES
CRGHIT = NO
CGIRST = NO