LeetCode 88 - Merge Sorted Array

LeetCode 88 - Merge Sorted Array

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

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively.

Merge nums1 and nums2 into a single array sorted in non-decreasing order.

The final sorted array should not be returned by the function, but instead be stored inside the array nums1. To accommodate this, nums1 has a length of m + n, where the first m elements denote the elements that should be merged, and the last n elements are set to 0 and should be ignored. nums2 has a length of n.

Example 1:

Input: nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3
Output: [1,2,2,3,5,6]
Explanation: The arrays we are merging are [1,2,3] and [2,5,6].
The result of the merge is [1,2,2,3,5,6] with the underlined elements coming from nums1.
Example 2:

Input: nums1 = [1], m = 1, nums2 = [], n = 0
Output: [1]
Explanation: The arrays we are merging are [1] and [].
The result of the merge is [1].
Example 3:

Input: nums1 = [0], m = 0, nums2 = [1], n = 1
Output: [1]
Explanation: The arrays we are merging are [] and [1].
The result of the merge is [1].
Note that because m = 0, there are no elements in nums1. The 0 is only there to ensure the merge result can fit in nums1.

Constraints:

nums1.length == m + n
nums2.length == n
0 <= m, n <= 200
1 <= m + n <= 200
-109 <= nums1[i], nums2[j] <= 109

Follow up: Can you come up with an algorithm that runs in O(m + n) time?

双指针法

import java.util.Arrays;
class Solution {
    public void merge(int[] nums1, int m, int[] nums2, int n) {
        int k = m+n;
        for(int idx=k-1, nums1idx=m-1, nums2idx=n-1; idx >=0; --idx){
            if(nums1idx < 0){ /* nums1 is finished. To get an element from nums2 directly is fine */
                nums1[idx] = nums2[nums2idx--];
            }else if(nums2idx < 0){ /* nums2 is finished. To get an element from nums1 directly is fine */
                nums1[idx] = nums1[nums1idx--];
            }else if(nums1[nums1idx] < nums2[nums2idx]){
                nums1[idx] = nums2[nums2idx--];
            }else{
                nums1[idx] = nums1[nums1idx--];
            }
        }
    }

    // public void merge(int[] nums1, int m, int[] nums2, int n) {
    //     int k = m+n;
    //     int[] temp = new int[m+n];
    //     for(int idx=0, nums1Idx=0, nums2Idx=0; idx < k; ++idx){            
    //         if(nums1Idx >= m){ /* nums1 is finished. To get an element from nums2 directly is fine */
    //             temp[idx] = nums2[nums2Idx++];
    //         }else if(nums2Idx >= n){ /* nums2 is finished. To get an element from nums1 directly is fine */
    //             temp[idx] = nums1[nums1Idx++];
    //         }else if(nums1[nums1Idx] < nums2[nums2Idx]){
    //             temp[idx] = nums1[nums1Idx++];
    //         }else{
    //             temp[idx] = nums2[nums2Idx++];
    //         }            
    //     }
    //     System.arraycopy(temp,0,nums1,0,k);
    // }

    // public void merge(int[] nums1, int m, int[] nums2, int n) {
    //     for( int i=0; i<n; ++i){
    //         nums1[m+i] = nums2[i];
    //     }
    //     Arrays.sort(nums1);
    // }
}