java用BigO表示法计算运行时间

Question

1 int sum=0;
2 long start = System.currentTimeMillis();
3 for (int i = 1; i <= N; i++) {
4 for (int j = 1; j <= N; j++) {
5 sum=sum+1;}}
6 long stop = System.currentTimeMillis();
7 long elapsed = (long)(stop - start);

我被这个问题困住了，我知道行1,2,5,6 and 7是以O(1)恒定时间运行的原始操作。我对循环有疑问，我想是O(n^2）谁能详细解释一下谢谢