String `s1` is anagram with `s2` if and only if they have the same number
of each letters. So we could calculate how many differences are there in the
count of each letters. One change would reduce 2 differences, so we could get
the total number of change by only adding the negative difference.

```
from collections import defaultdict
class Solution:
def minSteps(self, s: str, t: str) -> int:
count = defaultdict(int)
for c in s:
count[c] += 1
for c in t:
count[c] -= 1
steps = 0
for v in count.values():
if v > 0:
steps += v
return steps
```