The concise version:

```
class Solution(object):
def isSubsequence(self, s, t):
i, j = 0, 0
while i<len(s) and j<len(t):
i += s[i]==t[j]
j += 1
return i==len(s)
```

The fast version (10 times faster, but still pretty concise):

```
class Solution(object):
def isSubsequence(self, s, t):
for c in s:
i = t.find(c)
if i == -1:
return False
t = t[i+1:]
return True
```