53, 55, 56

1 files changed, 26 insertions, 0 deletions

diff --git a/euler53.c b/euler53.c
new file mode 100644
index 0000000..3e71c87
--- /dev/null
+++ b/euler53.c
@@ -0,0 +1,26 @@
+#include <stdio.h>
+
+/* number of r s.t. C(n, r)>x */
+int nr_ncr_bigger(int n, int x)
+{
+	int prod = 1;
+	int r = 0;
+
+	while (prod <= x && r*2 < n) {
+		r++;
+		prod = prod * (n-r+1) / r;
+	}
+
+	if (prod <= x)
+		return 0;
+	else
+		return n - r*2 + 1;
+}
+
+int main()
+{
+	int cnt = 0;
+	for (int i = 1; i <= 100; i++)
+		cnt += nr_ncr_bigger(i, 1000000);
+	printf("%d\n", cnt);
+}