Replace and remove

1class Solution:
2    def replaceAndRemove(self, size, s):
3        if len(s) == 0: return s
4        # a b a c _
5        # a a c _ _
6        # a a _ _ c
7        # a a d d c
8        # d d d d c
9        # remove b first
10        i, a = 0, 0
11        for j in range(size):
12            if s[j] == "a": a += 1
13            if s[j] != "b":
14                s[i] = s[j]
15                i += 1
16        
17        curr = i - 1
18        i += a - 1
19        res = i + 1
20        while curr >= 0:
21            if s[curr] == "a":
22                s[i - 1:i + 1] = "dd"
23                i -= 2
24            elif s[curr] != "a":
25                s[i] = s[curr]
26                i -= 1
27            curr -= 1
28
29        return res
30        
31