time complexity O(n), and no extra memory
IntuitionWe do need to use extra space because all digits has to sum up and the result should be zero, as long as even digits is positive and odd digits are neg
# Intuition
We do need to use extra space because all digits has to sum up and the result should be zero, as long as even digits is positive and odd digits are negative

# Approach
Recursivily return the next digit with the correct signal (positive or negative) to sum up all digits at the and

# Complexity
- Time complexity:
O(n)

- Space complexity:
we do not need extra space, O(1)


# Code
```python3 []
class Solution:
    def isBalanced(self, num: str) -> bool:

        def helper(i):
            if i >= len(num): return 0

            if i % 2 == 0:
                return (+1 * int(num[i])) + helper(i + 1)
            else:
                return (-1 * int(num[i])) + helper(i + 1)
        
        return helper(0) == 0
```
['Python3']