283 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.

For example, given nums = [0, 1, 0, 3, 12], after calling your function, nums should be [1, 3, 12, 0, 0].

Note: You must do this in-place without making a copy of the array. Minimize the total number of operations.

class Solution {
public:
    void moveZeroes(vector<int>& nums) {
        int size = nums.size();
        for (int i = 0; i < size; i++)
        {
            if (nums.at(i) == 0)
            {
                nums.push_back(0);
                nums.erase(nums.begin()+i);
                size--; i--;
            }
        }
    }
};

Implementation 2:

The Idea: We can solve this problem purely using iterators and swaps. The general idea is to swap each zero with the first instance of the next number, given that the zero follows the number. Both iterators begin at 0. We continue to increment the left iterator until we find a zero, do so the same with the number iterator. Given that the number and the zero is found and that the zero follows the number, we can swap the two. This way, we preserve the order of the numbers

Complexity: O(n) time (since both iterators do a single scan through the array at most) and O(1) space

class Solution:
    def moveZeroes(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """

        if not nums:
            return

        zero_i = 0
        number_i = 0
        size = len(nums)

        # idea: swap with the first instance of the next number
        while True:
            # increment to next zero
            while zero_i < size and nums[zero_i] != 0:
                zero_i += 1

            # implies that we scanned and didnt find zero
            if zero_i >= size: 
                return

            # increment to the next number
            while number_i < size and nums[number_i] == 0:
                number_i += 1

            # implies that we scanned and didnt find number
            if number_i >= size:
                return

            # number is in front of the zero, so just move on
            # because a swap wont bring the zero forward
            if zero_i > number_i:
                number_i += 1
                continue

            # swap
            nums[zero_i], nums[number_i] = nums[number_i], nums[zero_i]