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9.1
You are given two sorted arrays, A and B, and A has a large enough buffer at the end
to hold B. Write a method to merge B into A in sorted order.
First let‘s take a look at how merge two arrays works in merge sort algorithm;
public int[] mergeTwoArrays(int[] A, int[] B) {
int Alength = A.length;
int Blength = B.length;
int i = 0;
int j = 0;
int k = 0;
int[] C = new int[Alength + Blength];
// note integer can not be null; so can not write A[i] != null
while (i < Alength && j < Blength) {
if (A[i >= B[j]) {
C[k++] = B[j++];
}
else {
C[k++] = A[i++];
}
}
while (i < Alength) {
C[k++] = A[i++];
}
while (j < Blength) {
C[k++] = B[j++];
}
return C;
}
Next is merge two sorted list
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null)
return l2;
if (l2 == null)
return l1;
ListNode prehead = new ListNode(0);
// point to record the current position in the result;
ListNode point = prehead;
ListNode curr1 = l1;
ListNode curr2 = l2;
while (curr1 != null && curr2 != null) {
if (curr1.val < curr2.val) {
ListNode temp = curr1.next;
point.next = curr1;
point = curr1;
curr1 = curr1.next;
}
else {
point.next = curr2;
point = curr2;
curr2 = curr2.next;
}
}
while (curr1 != null) {
point.next = curr1;
}
while (curr2 != null) {
point.next = curr2;
}
return prehead;
}
Back to this question, it‘s a special case of merge two sorted array in which array A is much larger than array B. Therefore we don‘t need extra space to store the merged array. The strategy is to compare from then end to the front.
public void merge(int[] A, int m, int[] B, int n) {
// two points m and 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] = A[n - 1];
n--;
}
}
while (n > 0) {
A[m + n - 1] = A[n - 1];
n--;
}
}
9.1