https://leetcode.com/problems/merge-sorted-array/description/
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.
1 /*
2 * method 1: not good enough: used extra space!
3 * time: o(m+n)
4 * space: o(m+n): one calling stack but with extra space
5 * 从前往后弄，需要额外空间
6 * */
7 public void merge(int[] nums1, int m, int[] nums2, int n) {
8 int[] res = new int[m+n] ;
9 int i = 0 ;
10 int j = 0 ;
11 int index = 0 ;
12 //first merge into the res and then copy to the nums1: o(m+n)
13 while(i<=m-1 && j<=n-1){
14 if (nums1[i]<=nums2[j]){
15 res[index++] = nums1[i++] ;
16 } else{
17 res[index++] = nums2[j++];
18 }
19 }
20 //one side left corner case: very common for two pointers
21 while(i<=m-1){
22 res[index++] = nums1[i++] ;
23 }
24 while(j<=n-1){
25 res[index++] = nums2[j++];
26 }
27 //copy from res to nums1
28 for (int k = 0; k < res.length; k++) {
29 nums1[k] = res[k];
30 }
31 }
32
33 /* time(m+n) space: o(1)
34 * nums1{3,4,5, , }
35 * nums2{1,2}
36 * 从后往前弄就不需要额外的空间了， 很巧妙
37 * */
38 public void merge2(int[] nums1, int m, int[] nums2, int n){
39 int i = m-1 ;
40 int j = n -1 ;
41 int k = m+n-1;
42 while(i>=0 && j>=0){
43 nums1[k--] = nums1[i]>=nums2[j]? nums1[i--] : nums2[j--] ;
44 }
45 // {3,4,3,4,5} 这种情况，I 已经到最左边了 需要把J 复写在上面
46 while(i>=0){
47 nums1[k--] = nums1[i--];
48 }
49 while(j>=0){
50 nums1[k--] = nums2[j--] ;
51 }
52 }