二分查找

#include "stdio.h"

int bsearch (int *A,int x,int y,int v) {
int m;
while (x < y) {//[x,y)
m = x + (y - x) /2;
//	printf ("[%d,%d)=%d\n",x,y,m);
if (A[m] == v) {
return m;
}else if (A[m] > v) {
y = m;
}else {
x = m + 1;
}
}
return -1;
}
//求上界
//当v存时，返回第一个它出现的位置，否则返回一个下标i，在i处插入v([i],[i+1]...向后移动一个位置)
int lower_bound (int *A,int x,int y,int v) {
int m;
while (x < y) {
m = x + (y-x) / 2;
if (A[m] >= v) {
y = m;
}else {
x = m + 1;
}
}
return x;
}

//求下界
int upper_bound (int *A,int x,int y,int v) {
int m;
while (x < y) {
m = x + ( y-x ) / 2;
if (A[m] <= v) {
x = m + 1;
}else {
y = m;
}
}
return x;
}

//lower_bound的返回值为L，upper_bound的返回值为R，则出现v的子序列为[L,R)，当L=R的时候区间为空。
int main () {
int A[] = {-1,1,1,1,3};
int L,R;
int index = bsearch (A,0,5,1);
printf("%d\n",index);
L = lower_bound (A,0,5,1);
printf ("%d\n",L);
R = upper_bound (A,0,5,1);
printf ("%d\n",R);
printf ("the area of v is[%d,%d)",L,R);
return 0;
}
/*output:
*2
*1
*4
*the area of v is[1,4)
*/

#include "stdio.h"
#define MAX 200
int num[MAX];
int num2[MAX];
int lower_bound (int *A,int x,int y,int v) {
int m;
while (x < y) {
m = x + (y-x) / 2;
if (A[m] >= v) {
y = m;
}else {
x = m + 1;
}
}
return x;
}

//求下界
int upper_bound (int *A,int x,int y,int v) {
int m;
while (x < y) {
m = x + ( y-x ) / 2;
if (A[m] <= v) {
x = m + 1;
}else {
y = m;
}
}
return x;
}

//排序
void sort (int *A,int x,int y,int *B) {
if (y - x == 1) {
return;
}else {
int m = x + (y - x) / 2;
int p = x;
int q = m;
int i = 0;
sort (A,x,m,B);
sort (A,m,y,B);
printf ("[%d %d)\n",x,y);
while (p < m || q < y) {
if ( q < y &&( p >= m || A[q] <= A[p])) {
B[i++] = A[q++];
}else {
B[i++] = A[p++];
}
}
q = 0;
for (i = x;i < y;i++) {
A[i] = B[q++];
}
}
}

int main () {
int n,m;
int i;
int a,b;
int result;
scanf ("%d%d",&n,&m);
for (i = 0;i < n;i++) {
scanf ("%d",num+i);
}
sort (num,0,n,num2);
for (i = 0;i < n;i++) {
printf ("%d ",num[i]);
}
printf ("\n");
for (i = 0; i < m;i++) {
scanf ("%d%d",&a,&b);
result = upper_bound (num,0,n,b) - lower_bound(num,0,n,a);
printf ("%d\n",result);
}
return 0;
}