根据给定索引对数组重新排序 [英] Reorder array according to given index
问题描述
算法根据给定索引对数组重新排序
Algorithm reorder array according to given index
a[] = [50, 40, 70, 60, 90]
index[] = [3, 0, 4, 1, 2]
a= [60,50,90,40,70]
在 O(n) 中并且没有额外的数组/空格
in O(n) and With out extra array/spaces
推荐答案
您将需要用于临时变量和循环计数器/索引的空间.根据算法通常的重新排序"也会将 index[] 改回 {0, 1, 2, 3, 4}.
You'll need space for a temp variable and loop counters / indices. The usual "reorder" according to algorithm is also going to change index[] back to {0, 1, 2, 3, 4}.
提示,注意 index[] 中索引的顺序.
Hint, noting the ordering of indices in index[].
{0, 1, 2, 3, 4}
index[] = {3, 0, 4, 1, 2}
可以按照循环"进行重新排序.从 index[0] 开始,如果你查看 index[0],然后查看 index[index[0]],请注意循环",依此类推...
The reordering can be done by following the "cycles". Start with index[0], and note the "cycles" if you look at index[0], then index[index[0]], and so on ...
// 1st cycle
index[0] == 3 // cycle starts at 0
index[3] == 1
index[1] == 0 // end of cycle since back at 0
// 2nd cycle
index[2] == 4 // cycle starts at 2
index[4] == 2 // end of cycle since back at 2
示例 C 代码:
#include <stdio.h>
static int A[] = {50, 40, 70, 60, 90};
static int I[] = {3, 0, 4, 1, 2};
int main()
{
int i, j, k;
int tA;
/* reorder A according to I */
/* every move puts an element into place */
/* time complexity is O(n) */
for(i = 0; i < sizeof(A)/sizeof(A[0]); i++){
if(i != I[i]){
tA = A[i];
j = i;
while(i != (k = I[j])){
A[j] = A[k];
I[j] = j;
j = k;
}
A[j] = tA;
I[j] = j;
}
}
for(i = 0; i < sizeof(A)/sizeof(A[0]); i++)
printf("%d
", A[i]);
return 0;
}
相同的算法,但使用交换而不是移动(这是较慢的方法).
The same algorithm, but using swaps instead of moves (this is slower method).
#include <stdio.h>
#define swap(a, b) {(a)^=(b); (b)^=(a); (a)^=(b);}
static int A[] = {50, 40, 70, 60, 90};
static int I[] = {3, 0, 4, 1, 2};
int main()
{
int i, j, k;
/* reorder A according to I */
/* every swap puts an element into place */
/* last swap of a cycle puts both elements into place */
/* time complexity is O(n) */
for(i = 0; i < sizeof(A)/sizeof(A[0]); i++){
if(i != I[i]){
j = i;
while(i != (k = I[j])){
swap(A[j], A[k]);
I[j] = j;
j = k;
}
I[j] = j;
}
}
for(i = 0; i < sizeof(A)/sizeof(A[0]); i++)
printf("%d
", A[i]);
return 0;
}
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