Test-3 s = "zzzzz" t = "zzzzzzz" k = 4 , how come the answer should be "Yes"? I mean how it is possible to convert s into t with exactly 4 operations? That is if we perform two append-operations and two delete-operations, with 4 operations the strings won't be same. On the otherhand if we perform four append-operations and two delete-operations the strings will be same but number of operations > k, Thus, probably the test is not correct.

Java solution: public static String appendAndDelete(String s, String t, int k) {

int stringSLength = s.length();
int stringTLength = t.length();
int difference = Math.abs(stringSLength - stringTLength);

if(t.charAt(0) != s.charAt(0)){
    if(stringSLength + stringTLength <= k){
        return "Yes";
    }
    else{
        return "No";
    }
}
else{

    if(difference == 0 && !s.equals(t)){
        for(int i = 0; i < stringSLength; i++){
            if (s.charAt(stringSLength - 1 - i) != t.charAt(stringTLength - 1 - i)) {
                difference = i;
                break;
            }
        }
    }

    if(difference <= k){
        if(difference % 2 != 0 && k % 2 == 0){
            return "No";
        }

        return "Yes";
    }
    else{
        return "No";

    }
}

}

I think the tests are broken

why s=y t="yu " or t="yu" k=2 should return "NO" in the 5th test case ?

Task is totally wrong. You always can delete original string and then infinitely delete emty string. Result always should be "Yes".