added binarysearch.java

lessons/binarysearch.java

@@ -0,0 +1,27 @@
+// CODE:
+
+class Solution {
+    public int search(int[] nums, int target) {
+
+int start = 0;
+        int end = nums.length - 1;
+        while(start <= end) {
+            int mid = start + (end - start)/2;
+            if(nums[mid] == target) {
+                return mid;
+            }
+            else if(target < nums[mid]) {
+                end = mid - 1;
+            }
+            else {
+                start = mid + 1;
+            }
+        }
+        return -1;
+    }
+}
+
+
+// Input: nums = [-1,0,3,5,9,12], target = 9
+// Output: 4
+// Explanation: 9 exists in nums and its index is 4