class Solution {
    public void merge(int[] nums1, int m, int[] nums2, int n) {
        // Checking edge case: if nums2 is empty, we can just return whatever nums1 is, since it's already sorted.
        if (n == 0) {
            return;
        }
        // Checking edge case: if nums1 is empty, we just copy all nums2 values into nums1
        if (m == 0) {
            for(int i = 0; i < n; i++) {
                nums1[i] = nums2[i];
            }
            return;
        }

        // Approach: we're going backwards since we know the end is empty, and the two arrays are already sorted.
        int nums1Index = m-1;
        int nums2Index = n-1;
        int sortedArrayIndex = m + n -1;

        while (nums1Index >= 0 && nums2Index >= 0) {
            if(nums1[nums1Index] < nums2[nums2Index]) {
                nums1[sortedArrayIndex--] = nums2[nums2Index--];
            } else {
                nums1[sortedArrayIndex--] = nums1[nums1Index--];
            }
        }

        // Going through leftover values.
        while (nums1Index >= 0) {
            nums1[sortedArrayIndex--] = nums1[nums1Index--];
        }
        while (nums2Index >= 0) {
            nums1[sortedArrayIndex--] = nums2[nums2Index--];
        }
    }
}