diff --git a/800-900q/856.py b/800-900q/856.py
new file mode 100644
index 0000000..97b10bd
--- /dev/null
+++ b/800-900q/856.py
@@ -0,0 +1,33 @@
+'''
+Let's call an array A a mountain if the following properties hold:
+
+A.length >= 3
+There exists some 0 < i < A.length - 1 such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
+Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1].
+
+Example 1:
+
+Input: [0,1,0]
+Output: 1
+Example 2:
+
+Input: [0,2,1,0]
+Output: 1
+Note:
+
+3 <= A.length <= 10000
+0 <= A[i] <= 10^6
+A is a mountain, as defined above.
+'''
+class Solution(object):
+    def peakIndexInMountainArray(self, A):
+        """
+        :type A: List[int]
+        :rtype: int
+        """
+        m_index, pre_elem = 0, A[0]
+        for index in range(1, len(A)):
+            if A[index] > pre_elem:
+                pre_elem = A[index]
+            else:
+                return index-1