leetcode-88-merge-sorted-array.swift

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https://leetcode.com/problems/merge-sorted-array/


Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.

Note:

The number of elements initialized in nums1 and nums2 are m and n respectively.
You may assume that nums1 has enough space (size that is greater or equal to m + n) to hold additional elements from nums2.
Example:

Input:
nums1 = [1,2,3,0,0,0], m = 3
nums2 = [2,5,6],       n = 3

Output: [1,2,2,3,5,6]

*/

/**
Big O Annotation
Time complexity O(mn) we're running linear m * n through the 2 pointer approach.
Space complexity O(n) we're not using any extra space.
*/
func merge(_ nums1: inout [Int], _ m: Int, _ nums2: [Int], _ n: Int) {
	
    /**
    Create each a pointer per array
    and keep track of the insertions.
    */
    var pointer2: Int = 0 
    var pointer1: Int = 0 
    var insertions: Int = 0 

    while pointer2 < n {

        /**
        Get element of the 2nd array 
        for the current pointer
        */
        let element2 = nums2[pointer2]

        while true {

            // And the element of the 1st array
            let element1 = nums1[pointer1]

            /**
            Compare both elements
            */
            if element2 <= element1 {
            
                /**
                We need to:
                
                - insert at pointer of array 1
                - we need to pop one of the zeros
                - we need to update the insertions counter
                - and move to the next index of the array 1
                - and break the loop to move forward in array 2
                */
                nums1.insert(element2, at: pointer1)
                nums1.popLast()
                insertions += 1
                pointer1 += 1
                break

            /**
            If the pointer is larger than elements 
            available in the first array PLUS the insertions,
            we need to replace one of the remaining zeros 
            with the new element from array 2.
            */
            } else if pointer1 >= (m + insertions)  {

                nums1[pointer1] = element2
                pointer1 += 1
                break

            /**
            Else we just want to move the 
            pointer of array 1 along
            */
            } else {

                pointer1 += 1
            }
        }

        // And lastly move the pointer of array 2 forward
        pointer2 += 1
    }
}