資料結構與演算法(A)

(5 points) Consider an integer array $A$ of size $n > 0$ with elements $A[1], \dots, A[n]$, where the first $p$ elements are positive and the other $(n - p)$ elements are negative. The following algorithm calculates $p$ correctly by calling $\text{COMPUTE-P}(A, n)$. What loop invariant is maintained for the while loop of the algorithm?

COMPUTE-P(A, len)
1  if A[1] < 0 return 0
2  left = 1, right = len
3  while left < right
4      m = ceiling((left + right) / 2)
5      if A[m] > 0
6          left = m
7      else
8          right = m - 1
9  return left