if the numbers are in sequence, this function works fine. if its not in sequence its different story.

main method(){
int[] arrayofMillionNumbers = new int[100000];
for(int i = 1 ; i< arrayofMillionNumbers+1 ; i++){
arrayofMillionNumbers.add(i);
}
findMissing(arrayofMillionNumbers);
}
private int findMissing(int[] num) {
int missingNumber = 0;
for (int i = 0; i < (num.length - 1); i++) {
if ((num[i + 1] - num[i]) > 1) {
missingNumber = num[i] + 1;
break;
}
}
return missingNumber;
}