Update binary_search.py (#902)

* Update binary_search.py

* Update first_occurrence.py

Author: Anujeet23

algorithms/search/binary_search.py

@@ -34,18 +34,19 @@ def binary_search(array, query):
             high = mid - 1
     return None
 
+#In this below function we are passing array, it's first index , last index and value to be searched
 def binary_search_recur(array, low, high, val):
     """
     Worst-case Complexity: O(log(n))
 
     reference: https://en.wikipedia.org/wiki/Binary_search_algorithm
     """
-
-    if low > high:       # error case
+#Here in Logic section first we are checking if low is greater than high which means its an error condition because low index should not move ahead of high index
+    if low > high:       
         return -1
-    mid = (low + high) // 2
-    if val < array[mid]:
-        return binary_search_recur(array, low, mid - 1, val)
+    mid = low + (high-low)//2   #This mid will not break integer range
+    if val < array[mid]:  
+        return binary_search_recur(array, low, mid - 1, val) #Go search in the left subarray
     if val > array[mid]:
-        return binary_search_recur(array, mid + 1, high, val)
+        return binary_search_recur(array, mid + 1, high, val) #Go search in the right subarray
     return mid

algorithms/search/first_occurrence.py

@@ -11,7 +11,7 @@ def first_occurrence(array, query):
 
     low, high = 0, len(array) - 1
     while low <= high:
-        mid = (low + high) // 2
+        mid = low + (high-low)//2 #Now mid will be ininteger range
         #print("lo: ", lo, " hi: ", hi, " mid: ", mid)
         if low == high:
             break