Merge Sorted Array leetcode java（回顧MergeTwoArray和MergeTwoLinkedList）

題目：

Given two sorted integer arrays A and B, merge B into A as one sorted array.

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

 

題解：

這道題是說讓B merge到 A 裡面。

先複習下原本我們在MergeSort裡面怎麼利用一個新建的數量來merge two array：

程式碼如下：

 1 public int[] mergeTwoList(int[] A, int[] B) {
 2     int[] C = new int[A.length + B.length];
 3     int k = 0;
 4     int i = 0;
 5     int j = 0;
 6     while(i < A.length && j < B.length) {
 7         if (A[i] < B[j])
 8             C[k++] = A[i++];
 9         else
10             C[k++] = B[j++];
11     }
12     while (i < A.length) 
13         C[k++] = A[i++];
14     while (j < B.length) 
15         C[k++] = B[j++];
16     return C;
17 }

 

然後我們再順便複習下，怎麼merge two linked list，程式碼如下：

 1     public ListNode mergeTwoLists(ListNode leftlist, ListNode rightlist){
 2         if(rightlist == null)
 3             return leftlist;
 4         if(leftlist == null)
 5             return rightlist;
 6         
 7         ListNode fakehead = new ListNode(-1);
 8         ListNode ptr = fakehead;
 9         while(rightlist!=null&&leftlist!=null){
10             if(rightlist.val<leftlist.val){
11                 ptr.next = rightlist;
12                 ptr = ptr.next;
13                 rightlist = rightlist.next;
14             }else{
15                 ptr.next = leftlist;
16                 ptr = ptr.next;
17                 leftlist = leftlist.next;
18             }
19         }
20         
21         if(rightlist!=null)
22             ptr.next = rightlist;
23         if(leftlist!=null)
24             ptr.next = leftlist;
25         
26         return fakehead.next;
27     }

 

可以看出merge的思路都是在從頭比較兩個list的value，用兩個指標分別指向當前要比較的node上面。而且最後都會處理下剩下的元素。

 

而這道題是不能借助一個新的array的，那麼我們就不好從前往後比了（不好插入位置）。方便的方法是從後往前比，然後最後處理剩下的元素。

程式碼如下：

     public void merge(int A[], int m, int B[], int n) {
        while(m > 0 && n > 0){
            if(A[m-1] > B[n-1]){
                A[m+n-1] = A[m-1];
                m--;
            }else{
                A[m+n-1] = B[n-1];
                n--;
            }
        }
 
        while(n > 0){
            A[m+n-1] = B[n-1];
            n--;
        }
    }