CountPair

public int countPair(int[] A, int X) {
  int small = 0;
  int big = 0;
  int count = 0;
  // find max big such that A[0] + A[big] <= X
  while (big + 1 < A.length && A[0] + A[big + 1] <= X) {
    big++;
  }

  // count use A[small] as smallest number, A[small + 1: big] as bigger number
  while (big > small) {
    if (A[small] + A[big] <= X) {
      count += big - small;
      small++;
    } else /* A[small] + A[big] > X */ {
      big--;
    }
  }
  return count;
}