我想写一个函数,它以一个字母数组作为参数,并从中选择一些字母。
假设您提供了一个包含8个字母的数组,并希望从中选择3个字母。那么你应该得到:
8! / ((8 - 3)! * 3!) = 56
数组(或单词),每个数组由3个字母组成。
我想写一个函数,它以一个字母数组作为参数,并从中选择一些字母。
假设您提供了一个包含8个字母的数组,并希望从中选择3个字母。那么你应该得到:
8! / ((8 - 3)! * 3!) = 56
数组(或单词),每个数组由3个字母组成。
当前回答
简短快速的c#实现
public static IEnumerable<IEnumerable<T>> Combinations<T>(IEnumerable<T> elements, int k)
{
return Combinations(elements.Count(), k).Select(p => p.Select(q => elements.ElementAt(q)));
}
public static List<int[]> Combinations(int setLenght, int subSetLenght) //5, 3
{
var result = new List<int[]>();
var lastIndex = subSetLenght - 1;
var dif = setLenght - subSetLenght;
var prevSubSet = new int[subSetLenght];
var lastSubSet = new int[subSetLenght];
for (int i = 0; i < subSetLenght; i++)
{
prevSubSet[i] = i;
lastSubSet[i] = i + dif;
}
while(true)
{
//add subSet ad result set
var n = new int[subSetLenght];
for (int i = 0; i < subSetLenght; i++)
n[i] = prevSubSet[i];
result.Add(n);
if (prevSubSet[0] >= lastSubSet[0])
break;
//start at index 1 because index 0 is checked and breaking in the current loop
int j = 1;
for (; j < subSetLenght; j++)
{
if (prevSubSet[j] >= lastSubSet[j])
{
prevSubSet[j - 1]++;
for (int p = j; p < subSetLenght; p++)
prevSubSet[p] = prevSubSet[p - 1] + 1;
break;
}
}
if (j > lastIndex)
prevSubSet[lastIndex]++;
}
return result;
}
其他回答
简短的java解决方案:
import java.util.Arrays;
public class Combination {
public static void main(String[] args){
String[] arr = {"A","B","C","D","E","F"};
combinations2(arr, 3, 0, new String[3]);
}
static void combinations2(String[] arr, int len, int startPosition, String[] result){
if (len == 0){
System.out.println(Arrays.toString(result));
return;
}
for (int i = startPosition; i <= arr.length-len; i++){
result[result.length - len] = arr[i];
combinations2(arr, len-1, i+1, result);
}
}
}
结果将是
[A, B, C]
[A, B, D]
[A, B, E]
[A, B, F]
[A, C, D]
[A, C, E]
[A, C, F]
[A, D, E]
[A, D, F]
[A, E, F]
[B, C, D]
[B, C, E]
[B, C, F]
[B, D, E]
[B, D, F]
[B, E, F]
[C, D, E]
[C, D, F]
[C, E, F]
[D, E, F]
#include <stdio.h>
unsigned int next_combination(unsigned int *ar, size_t n, unsigned int k)
{
unsigned int finished = 0;
unsigned int changed = 0;
unsigned int i;
if (k > 0) {
for (i = k - 1; !finished && !changed; i--) {
if (ar[i] < (n - 1) - (k - 1) + i) {
/* Increment this element */
ar[i]++;
if (i < k - 1) {
/* Turn the elements after it into a linear sequence */
unsigned int j;
for (j = i + 1; j < k; j++) {
ar[j] = ar[j - 1] + 1;
}
}
changed = 1;
}
finished = i == 0;
}
if (!changed) {
/* Reset to first combination */
for (i = 0; i < k; i++) {
ar[i] = i;
}
}
}
return changed;
}
typedef void(*printfn)(const void *, FILE *);
void print_set(const unsigned int *ar, size_t len, const void **elements,
const char *brackets, printfn print, FILE *fptr)
{
unsigned int i;
fputc(brackets[0], fptr);
for (i = 0; i < len; i++) {
print(elements[ar[i]], fptr);
if (i < len - 1) {
fputs(", ", fptr);
}
}
fputc(brackets[1], fptr);
}
int main(void)
{
unsigned int numbers[] = { 0, 1, 2 };
char *elements[] = { "a", "b", "c", "d", "e" };
const unsigned int k = sizeof(numbers) / sizeof(unsigned int);
const unsigned int n = sizeof(elements) / sizeof(const char*);
do {
print_set(numbers, k, (void*)elements, "[]", (printfn)fputs, stdout);
putchar('\n');
} while (next_combination(numbers, n, k));
getchar();
return 0;
}
Python中的简短示例:
def comb(sofar, rest, n):
if n == 0:
print sofar
else:
for i in range(len(rest)):
comb(sofar + rest[i], rest[i+1:], n-1)
>>> comb("", "abcde", 3)
abc
abd
abe
acd
ace
ade
bcd
bce
bde
cde
为了解释,递归方法用下面的例子描述:
示例:A B C D E 3的所有组合是:
A与其余2的所有组合(B C D E) B与其余2的所有组合(C D E) C与其余2的所有组合(D E)
下面是一个方法,它从一个随机长度的字符串中给出指定大小的所有组合。类似于昆玛斯的解,但适用于不同的输入和k。
代码可以更改为换行,即'dab'从输入'abcd' w k=3。
public void run(String data, int howMany){
choose(data, howMany, new StringBuffer(), 0);
}
//n choose k
private void choose(String data, int k, StringBuffer result, int startIndex){
if (result.length()==k){
System.out.println(result.toString());
return;
}
for (int i=startIndex; i<data.length(); i++){
result.append(data.charAt(i));
choose(data,k,result, i+1);
result.setLength(result.length()-1);
}
}
"abcde"的输出:
ABC abd ace ade BCD bce bde cde
我发现这个线程很有用,我想我会添加一个Javascript解决方案,你可以弹出到Firebug。取决于你的JS引擎,如果起始字符串很大,可能会花一点时间。
function string_recurse(active, rest) {
if (rest.length == 0) {
console.log(active);
} else {
string_recurse(active + rest.charAt(0), rest.substring(1, rest.length));
string_recurse(active, rest.substring(1, rest.length));
}
}
string_recurse("", "abc");
输出如下:
abc
ab
ac
a
bc
b
c