hiho 37 二分查找k小数

问题

解法

#include <bits/stdc++.h>
using namespace std;
int *a;
int partition(int L, int R)
{
int pos = L;
R++;
for(;;){
do{++L;} while(a[L] < a[pos] && L < R);
do{--R;} while(a[R] > a[pos] && R >= L);
if (L>=R)
break;
swap(a[L], a[R]);
}
swap(a[pos], a[R]);
return R;
}
int main()
{
int n, k;
scanf("%d %d", &n, &k);
a = new int
;
for (int i=0; i<n; ++i)
scanf("%d", a+i);
int L =0;
int R = n-1;
while(L<=R)
{
int m = partition(L, R);
if (m ==k-1){
printf("%d\n", a[m]);
return 0;
}else if (m < k-1){
L = m+1;
}else
R= m-1;
}
printf("-1\n");
return 0;
}