Merge Sorted Array

Problem

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

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

Solution

Reversed Index: O(n) time, O(1) space

public class Solution {
    public void merge(int[] nums1, int m, int[] nums2, int n) {
        int index = m + n - 1, i = m - 1, j = n - 1;
        while (i >= 0 && j >= 0) nums1[index--] = nums1[i] >= nums2[j] ? nums1[i--] : nums2[j--]; 
        while (j >= 0) nums1[index--] = nums2[j--];

    }
}

Analysis

We construct the merged array in reversed order and make the problem much easier
Notice that we don't have to check while (i >= 0) cause nums1 is the array we need to put sorted array in
FYI, to get a sub-array, use Arrays.copyOfRange(oldArray, startIndex, endIndex); ;)