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# Move Zeroes

Given an array `nums`, write a function to move all `0`'s to the end of it while maintaining the relative order of the non-zero elements.

**Example:**

```
Input: [0,1,0,3,12]
Output: [1,3,12,0,0]
```

####

#### Approach:

This problem could easily be implemented by making a copy of the array therefore we'll focus on a more efficient implementation. 

* We initialize two pointers `i = 0`, `j = 0`. Iterate `j` over `range(len(nums))`
* if `nums[j] != 0`, we swap `nums[i]` and `nums[j]`, and increment `i` by `1`.

We can now observe that the loop invariant nums\[:i+1] contains all nonzero elements in `nums[:j+1]` with relative order preserved. Hence when `j` reaches `len(nums)-1`, `nums[:i+1]` contains all nonzero elements in `nums` with relative order preserved.

```python
class Solution(object):
    def moveZeroes(self, nums):
        """
        :type nums: List[int]
        :rtype: None Do not return anything, modify nums in-place instead.
        """
        i = 0
        for j in range(len(nums)):
            if nums[j]:
                nums[i], nums[j] = nums[j], nums[i]
                i += 1
```

**`Time: O(n)                Space: O(1)`**