added java file

Author: Amrutha26

PeakIndex.java

@@ -0,0 +1,27 @@
+//https://leetcode.com/problems/peak-index-in-a-mountain-array/
+
+class Solution {
+    public int peakIndexInMountainArray(int[] arr) {
+        int start = 0;
+        int end = arr.length - 1;
+        
+        while(start < end) {
+            int mid = start + (end - start) / 2;
+            if(arr[mid] > arr[mid+1]) {
+                // you are in dec part of the array
+                // this may be the ans, but look at left
+                end = mid;
+            } else {
+                // arr[mid + 1] > arr[mid]
+                // you are in inc part of the array
+                start = mid + 1;
+                   }
+        } 
+        
+        // In the end, start == end and pointing to the largest number because of the 2 checks 
+        // start and end are always trying to find max element in the above 2 checks
+        // hence, when they are pointing to just one element, that is the max element
+        // At every point of time of start and end, there may be a best possible ans at that particular time
+        return end;
+    }
+}