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852-peak-index-in-a-mountain-array/852-peak-index-in-a-mountain-array.java

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+class Solution {
+    public int peakIndexInMountainArray(int[] arr) {
+        return search(arr, 0, arr.length-1);
+    }
+
+    private int search(int[] arr, int start, int end) {
+        while (start <= end) {
+            int mid = start + (end -start)/2;
+            if (mid == 0) {
+                start = mid+1;
+                continue;
+            }
+            if (mid == arr.length-1) {
+                end = mid-1;
+                continue;
+            }
+            if (arr[mid] > arr[mid+1] && arr[mid] > arr[mid-1]) {
+                return mid;
+            }
+            if (arr[mid] > arr[mid+1] && arr[mid] < arr[mid-1]) {
+                end = mid-1;
+            } else if (arr[mid] > arr[mid-1] && arr[mid] < arr[mid+1]) {
+                start = mid+1;
+            }
+        }
+        return -1;
+    }
+}