[7, 100, 83, 1000, 9, 100, 19]

[700, 83, 1000, 900, 19]

stack2 = []
i = 1
while i < len(stack):
    if stack[i] == 100:
        stack2.append(stack[i-1]*100)
    elif len(stack) == 2 and i == len(stack)-1:  # Гдето здесь ошибка есть
        stack2.append(stack[i-1])
        stack2.append(stack[i])
    elif i == len(stack)-1:
        stack2.append(stack[i])
    elif stack[i-1] != 100:
        stack2.append(stack[i-1])
    i += 1

from functools import reduce

data = [7, 100, 83, 1000, 9, 100, 19]

reduce(lambda acc, val: acc[:-1] + [acc[-1] * val] if val == 100 and len(acc) else acc + [val], data, [])

x = [7, 100, 83, 1000, 9, 100, 19]

for value, index in zip(reversed(x[1:]), range(len(x) - 1, 0, -1)):
    if (value == 100):
        x[index - 1] *= x.pop(index)

print(x)  # [700, 83, 1000, 900, 19]

lst = [7, 100, 83, 1000, 9, 100, 19]
lst2 = []
for i in range(len(lst)):
    if lst[i] == 100:
        continue
    if i != len(lst) - 1:
        if lst[i+1] == 100:
            lst2.append(lst[i] * 100)
        else:
            lst2.append(lst[i])
    else:
        lst2.append(lst[i])
print(lst2)