add numerical recipes library

lib/nr/cpp/recipes/select.cpp

@@ -0,0 +1,40 @@
+#include "nr.h"
+
+DP NR::select(const int k, Vec_IO_DP &arr)
+{
+	int i,ir,j,l,mid;
+	DP a;
+
+	int n=arr.size();
+	l=0;
+	ir=n-1;
+	for (;;) {
+		if (ir <= l+1) {
+			if (ir == l+1 && arr[ir] < arr[l])
+				SWAP(arr[l],arr[ir]);
+			return arr[k];
+		} else {
+			mid=(l+ir) >> 1;
+			SWAP(arr[mid],arr[l+1]);
+			if (arr[l] > arr[ir])
+				SWAP(arr[l],arr[ir]);
+			if (arr[l+1] > arr[ir])
+				SWAP(arr[l+1],arr[ir]);
+			if (arr[l] > arr[l+1])
+				SWAP(arr[l],arr[l+1]);
+			i=l+1;
+			j=ir;
+			a=arr[l+1];
+			for (;;) {
+				do i++; while (arr[i] < a);
+				do j--; while (arr[j] > a);
+				if (j < i) break;
+				SWAP(arr[i],arr[j]);
+			}
+			arr[l+1]=arr[j];
+			arr[j]=a;
+			if (j >= k) ir=j-1;
+			if (j <= k) l=i;
+		}
+	}
+}