I use an interesting method which transform * and / into + and - operation. The basic idea is that if all the operators have same priority, then what we do is simple: calculate from left to right according to the operators. So I transform a+b*c into a+0+x (say x = b*c).

```
def calculate(self, s):
"""
:type s: str
:rtype: int
"""
s = s.replace(' ', '')
# Store all number and operators in list.
num = []
numStr = []
op = []
i = 0
while i < len(s):
if s[i] >= '0' and s[i] <= '9':
numStr.append(s[i])
elif s[i]=='+' or s[i]=='-' or s[i]=='*' or s[i]=='/':
num.append(int(''.join(numStr)))
del numStr[:]
op.append(s[i])
i += 1
if len(numStr) != 0:
num.append(int(''.join(numStr)))
# Transform all a*b into 0+(a*b).
for i in range(len(op)):
if op[i] == '*':
num[i+1] = num[i] * num[i+1]
elif op[i] == '/':
num[i+1] = num[i] / num[i+1]
if op[i]=='*' or op[i]=='/':
num[i] = 0
if i != 0:
op[i] = op[i-1]
else:
op[i] = '+'
# Calculate + and - from left to right.
res = num[0]
for i in range(len(op)):
if op[i] == '+':
res += num[i+1]
elif op[i] == '-':
res -= num[i+1]
return res
```