randomized selection for any order statistic

misc/rselect.c

@@ -0,0 +1,54 @@
+#include <stdio.h>
+#include<stdlib.h>
+#include<time.h>
+void swap(int *a ,int *b)
+{int t;t =*a;*a=*b;*b=t;}
+int part(int a[],int l,int r,int n,int pivot,int pindex)
+{int p1=l,p2=r;
+    while(p2>p1)
+    {
+        if (a[p1] > pivot && a[p2]<pivot)
+        {swap(&a[p1],&a[p2]);}
+        else
+        {
+            if (a[p1] <=pivot)
+            {p1++;}
+            if (a[p2]>=pivot)
+            {p2--;}
+        }
+    }
+    swap(&a[pindex],&a[p2]);
+    return p2;
+}
+int rselect(int a[],int l,int r,int n,int o)
+{
+    int pivot,pindex,pactual;
+    if (r>l)
+    {
+    pindex = rand()%(r-l+1);
+    pivot = a[pindex];
+    pactual = part(a,l,r,n,pivot,pindex);
+    
+    if (pactual == o)
+    {return a[pactual];}
+    
+    if (o < pactual)
+    {rselect(a,l,pactual-1,n,o);}
+    
+    if (o>pactual)
+    {rselect(a,pactual+1,r,n,o-pactual);}
+    }
+    if (r==l)
+    {return a[l];}
+}
+int main()
+{srand(time(NULL));
+    int n,o,i,*a;
+   scanf("%d %d",&n,&o);
+   a = (int*)malloc(n*sizeof(int));
+   for (i=0;i<n;i++)
+   {scanf("%d",a+i);}
+printf("\n\n%d",rselect(a,0,n-1,n,o));
+    return 0;
+}
+