Given an array of integers nums, half of the integers in nums are odd, and the other half are even.
Sort the array so that whenever nums[i] is odd, i is odd, and whenever nums[i] is even, i is even.
Return any answer array that satisfies this condition.
Example 1:
Input: nums = [4,2,5,7]
Output: [4,5,2,7]
Explanation: [4,7,2,5], [2,5,4,7], [2,7,4,5] would also have been accepted.
Example 2:
Input: nums = [2,3]
Output: [2,3]
Constraints:
2 <= nums.length <= 2 * 104
nums.length is even.
Half of the integers in nums are even.
0 <= nums[i] <= 1000
Follow Up: Could you solve it in-place?
Solutions
Solution 1: Two Pointers
We use two pointers $i$ and $j$ to point to even and odd indices respectively.
When $i$ points to an even index, if $nums[i]$ is odd, then we need to find an odd index $j$ such that $nums[j]$ is even, and then swap $nums[i]$ and $nums[j]$. Continue to iterate until $i$ points to the end of the array.
The time complexity is $O(n)$, where $n$ is the length of the array. The space complexity is $O(1)$.