Added medium problem

Author: das-jishu

Medium/next-greater-element.py

@@ -0,0 +1,20 @@
+
+# NEXT GREATER ELEMENT
+
+# O(N) time and space
+def nextGreaterElement(array):
+    # Write your code here.
+	result = [-1] * len(array)
+	stack = [0]
+	
+	for i in range(2 * len(array)):
+		index = i % len(array)
+		if len(stack) > 0 and array[index] <= array[stack[-1]]:
+			stack.append(index)
+			continue
+		while stack and array[stack[-1]] < array[index]:
+			j = stack.pop()
+			result[j] = array[index]
+		stack.append(index)
+		
+	return result