Added binomial coefficient (#1467)

* Added binomial coefficient

* Format code with psf/black and add a doctest

maths/binomial_coefficient.py

@@ -0,0 +1,20 @@
+def binomial_coefficient(n, r):
+    """
+    Find binomial coefficient using pascals triangle.
+
+    >>> binomial_coefficient(10, 5)
+    252
+    """
+    C = [0 for i in range(r + 1)]
+    # nc0 = 1
+    C[0] = 1
+    for i in range(1, n + 1):
+        # to compute current row from previous row.
+        j = min(i, r)
+        while j > 0:
+            C[j] += C[j - 1]
+            j -= 1
+    return C[r]
+
+
+print(binomial_coefficient(n=10, r=5))