Algorithm to Swap Consecutive Pair of Even Numbers

Given a list of integers nums, swap each consecutive even indexes with each other, and swap each consecutive odd indexes with each other.

Example 1
Input

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8]
Output

[2, 3, 0, 1, 6, 7, 4, 5, 8]
Explanation

0 and 2 gets swapped
1 and 3 gets swapped
4 and 6 gets swapped
5 and 7 gets swapped
8 remains the same

Consecutive Pair Swapping Algorithm

We can group array elements into four, and do the Consecutive Pair Swapping. For the last group which is less than 4 but greater than 2, we can do one more swap.

vector<int> swapConsecutivePairs(vector<int>& nums) {
    int i = 0;
    for (; i + 3 < nums.size(); i += 4) {
        swap(nums[i], nums[i + 2]);
        swap(nums[i + 1], nums[i + 3]);
    }
    if (i + 2 < nums.size()) {
        swap(nums[i], nums[i + 2]);
    }
    return nums;
}

The time complexity is O(N) and the space requirement is O(1) constant.

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