알고리즘 도론 코드 제9장 중위수와 순서 통계학

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제9장 중위수와 순서통계학


9.1 최소값 및 최대값

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
int minimum(int A[], int n)
{
	int min = A[0];
	for (int i = 1; i < n; i++) {
		if (min > A[i]) {
			min = A[i];
		}
	}
	return min;
}

void min_and_max(int A[], int n, int *min, int *max)
{
	int i;
	if (n % 2 == 1) {
		*min = A[0];
		*max = A[0];
		i = 1;
	} else {
		if (A[0] > A[1]) {
			*max = A[0];
			*min = A[1];
		} else {
			*max = A[1];
			*min = A[0];
		}
		i = 2;
	}
	for (; i < n; i += 2) {
		if (A[i] > A[i + 1]) {
			if (A[i] > *max) {
				*max = A[i];
			}
			if (A[i + 1] < *min) {
				*min = A[i + 1];
			}
		} else {
			if (A[i + 1] > *max) {
				*max = A[i + 1];
			}
			if (A[i] < *min) {
				*min = A[i];
			}
		}
	}
}

void print_array(int a[], int n)
{
	for (int i = 0; i < n; i++) {
		printf("%d ", a[i]);
	}
	printf("
"); } int main() { srand((unsigned)time(NULL)); int a[10]; for (int i = 0; i < 10; i++) { a[i] = rand() % 100; } print_array(a,10); printf(" :%d
",minimum(a,10)); int min; int max; min_and_max(a,10,&min,&max); printf(" :%d,%d
",min,max); }

9.2 기대 선형 시간으로 선택

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
void swap(void *a, void *b, size_t elem_size)
{
	if(a==NULL||b==NULL||a==b)
		return;
	char temp[elem_size];	/*     */
	memcpy(temp, a, elem_size);
	memcpy(a, b, elem_size);
	memcpy(b, temp, elem_size);
}
int partition(void *base, size_t elem_size, int p, int r,
	      int (*comp) (const void *, const void *))
{
	char *cbase = base;
	void *key = &cbase[r * elem_size];
	int i = p - 1;
	for (int j = p; j < r; j++) {
		if (comp(&cbase[j * elem_size], key) <= 0) {
			++i;
			swap(&cbase[i * elem_size], &cbase[j * elem_size],
			     elem_size);
		}
	}
	swap(&cbase[(i + 1) * elem_size], key, elem_size);
	return i + 1;
}

int randomized_partition(void *base, size_t elem_size, int p, int r,
			 int (*comp) (const void *, const void *))
{
	char *cbase = base;
	int i = rand() % (r - p + 1) + p;
	swap(&cbase[r * elem_size], &cbase[i * elem_size], elem_size);
	return partition(base, elem_size, p, r, comp);
}

void *randomized_select(void *base, size_t elem_size, int p, int r, int i,
			int (*comp) (const void *, const void *))
{
	char *cbase = base;
	if (p == r)
		return &cbase[p * elem_size];
	int q = randomized_partition(base, elem_size, p, r, comp);
	int k = q - p + 1;
	if (i == k) {
		return &cbase[q * elem_size];
	} else if (i < k) {
		return randomized_select(base, elem_size, p, q - 1, i, comp);
	} else {
		return randomized_select(base, elem_size, q + 1, r, i - k,
					 comp);
	}
}

void print_array(int a[], int length)
{
	for (int i = 0; i < length; i++) {
		printf("%d ", a[i]);
	}
	printf("
"); } int cmp_int(const void *p1, const void *p2) { const int *pa = p1; const int *pb = p2; if (*pa < *pb) return -1; if (*pa == *pb) return 0; return 1; } void randomize_quick_sort(void *base, size_t elem_size, int p, int r, int (*comp) (const void *, const void *)) { if (p < r) { int q = randomized_partition(base, elem_size, p, r, comp); randomize_quick_sort(base, elem_size, p, q - 1, comp); randomize_quick_sort(base, elem_size, q + 1, r, comp); } } int main() { srand((unsigned)time(NULL)); int a[10]; for (int i = 0; i < 10; i++) { a[i] = rand() % 100; } printf(" :
"); print_array(a, 10); int order = 3; int *select_value = randomized_select(a, sizeof(int), 0, 9, order, cmp_int); randomize_quick_sort(a, sizeof(int),0,9, cmp_int); printf(" %d :%d
", order, *select_value); printf(" :%s
", *select_value == a[order - 1] ? " " : " "); return 0; }

9.3 최악의 경우 선형 시간의 선택

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
void insertion_sort(void *base, size_t elem_size, size_t n,
		    int (*comp) (const void *, const void *))
{
	char *cbase = base;
	char key[elem_size];
	for (size_t i = 1; i < n; i++) {
		memcpy(key, &cbase[i * elem_size], elem_size);
		/* base[i]       base[0..i-1]  */
		int j = i - 1;
		while (j >= 0 && comp(&cbase[j * elem_size], key) > 0) {
			memcpy(&cbase[(j + 1) * elem_size],
			       &cbase[j * elem_size], elem_size);
			j--;
		}
		memcpy(&cbase[(j + 1) * elem_size], key, elem_size);
	}
}

void swap(void *a, void *b, size_t elem_size)
{
	if (a == NULL || b == NULL || a == b)
		return;
	char temp[elem_size];	/*     */
	memcpy(temp, a, elem_size);
	memcpy(a, b, elem_size);
	memcpy(b, temp, elem_size);
}

int partition(void *base, size_t elem_size, int p, int r, void *pivot,
	      int (*comp) (const void *, const void *))
{
	char *cbase = base;
	void *key = pivot;
	int i = p - 1;
	int pivot_pos = p;	/*      */
	for (int j = p; j < r; j++) {
		if (comp(&cbase[j * elem_size], key) == 0)
			pivot_pos = j;	/*        */
		if (comp(&cbase[j * elem_size], key) <= 0) {
			++i;
			swap(&cbase[i * elem_size], &cbase[j * elem_size],
			     elem_size);
		}
	}
	swap(&cbase[(i + 1) * elem_size], &cbase[pivot_pos * elem_size],
	     elem_size);
	return i + 1;
}

void *select(void *base, size_t elem_size, int p, int r, int order,
	     int (*comp) (const void *, const void *))
{
	char *cbase = base;
	if (p == r)
		return &cbase[p * elem_size];
	int n = r - p + 1;
	int array_count = n % 5 == 0 ? n / 5 : n / 5 + 1;
	char array[elem_size * array_count];
	for (int i = 0; i < array_count; i++) {
		int begin = p + i * 5;
		int end = begin + 4 < r ? begin + 4 : r;
		insertion_sort(&cbase[begin * elem_size], elem_size,
			       end - begin + 1, comp);
		int middle=begin+(end-begin)/2;
		memcpy(&array[i * elem_size], &cbase[middle * elem_size],
		       elem_size);
	}
	void *x =
	    select(array, elem_size, 0, array_count - 1, (array_count + 1) / 2,
		   comp);
	/*         x     A,              */
	int q = partition(base, elem_size, p, r, x, comp);
	int k = q - p + 1;
	if (order == k) {
		return &cbase[q * elem_size];
	} else if (order < k) {
		return select(base, elem_size, p, q - 1, order, comp);
	} else {
		return select(base, elem_size, q + 1, r, order - k, comp);
	}
}

void print_array(int a[], int length)
{
	for (int i = 0; i < length; i++) {
		printf("%d ", a[i]);
	}
	printf("
"); } int cmp_int(const void *p1, const void *p2) { const int *pa = p1; const int *pb = p2; if (*pa < *pb) return -1; if (*pa == *pb) return 0; return 1; } int main() { srand((unsigned)time(NULL)); int a[10]; for (int i = 0; i < 10; i++) { a[i] = rand() % 100; } printf(" :
"); print_array(a, 10); int order = 3; int *select_value = select(a, sizeof(int), 0, 9, order, cmp_int); insertion_sort(a, sizeof(int), 10, cmp_int); printf(" %d :%d
", order, *select_value); printf(" :%s
", *select_value == a[order - 1] ? " " : " "); return 0; }

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