MOD = 10**9 + 7
def count_subsequence_inversions(a):
n = len(a)
if n < 2:
return 0
# 离散化数组
sorted_a = sorted(set(a))
mapping = {val: idx + 1 for idx, val in enumerate(sorted_a)}
arr = [mapping[x] for x in a]
m = len(sorted_a)
# 树状数组实现
tree = [0] * (m + 1)
def update(index, delta):
while index <= m:
tree[index] += delta
index += index & -index
def query(index):
s = 0
while index:
s += tree[index]
index -= index & -index
return s
inversion_count = 0
# 从后向前遍历，统计逆序对
for i in range(n - 1, -1, -1):
x = arr[i]
if x > 1:
inversion_count += query(x - 1)
update(x, 1)
# 计算 2^(n-2) mod MOD
power = pow(2, n - 2, MOD)
total_inversions = inversion_count * power % MOD
return total_inversions
