Finding "crossover" index with modified binary search algorithm in Python

Question

The task is to find "crossover" index of two arrays. The crossover index is the index, that for array x and y:
assert(x[left] > y[left])
assert(x[right] < y[right])

I am supposed to use recursion to solve this problem.
Test case to pass are:

Test 1:
x, y = [0, 1, 2, 3, 4, 5, 6, 7], [-2, 0, 4, 5, 6, 7, 8, 9]

Test 2:
x, y = [0, 1, 2, 3, 4, 5, 6, 7], [-2, 0, 4, 4.2, 4.3, 4.5, 8, 9]

Test 3:
x, y = [0, 1], [-10, 10]

Test 4:
x, y = [0, 1, 2, 3], [-10, -9, -8, 5]

I modified the binary search algorithm. Below is my code:

def findCrossoverIndexHelper(arr_x, arr_y, left, right):
    if len(x) == len(y) and 0 <= left <= right - 1 and right < len(x):
        mid = (left + right) // 2
        if arr_x[mid] >= arr_y[mid] and arr_x[mid + 1] < arr_y[mid + 1]:
            print("This executes")
            return mid
        elif arr_x[mid] < arr_y[mid] and arr_x[mid + 1] > arr_y[mid + 1]:
            print("This executes 1")
            return findCrossoverIndexHelper(arr_x, arr_y, mid + 1, right)
        else:
            print("This executes 2")
            return findCrossoverIndexHelper(arr_x, arr_y, left, mid - 1)

My code passes Test cases 1, 2 and 3, but it can’t pass 4.

Do you have any suggestions where I am wrong or what I miss?

Thank you!

Asked By: Ralph L

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Source

Answer 1

The following function should work:

def findCrossoverIndexHelper(arr_x, arr_y, left, right):
    if len(x) == len(y) and 0 <= left <= right - 1 and right < len(x):
        mid = (left + right) // 2
        if arr_x[mid] >= arr_y[mid] and arr_x[mid + 1] < arr_y[mid + 1]:
            return mid
        elif arr_x[mid] < arr_y[mid] and arr_x[mid + 1] > arr_y[mid + 1]:
            return findCrossoverIndexHelper(arr_x, arr_y, mid + 1, right)

        elif arr_x[mid] > arr_y[mid] and arr_x[mid + 1] > arr_y[mid + 1]:
            return findCrossoverIndexHelper(arr_x, arr_y, mid + 1, right)
        else:
            return findCrossoverIndexHelper(arr_x, arr_y, left, mid - 1)

Answered By: SHARUKH

Answer 2

for me this code worked

def findCrossoverIndexHelper(x, y, left, right):
    if len(x) == len(y) and 0 <= left <= right - 1:
        mid = (left + right) // 2
        if x[mid] >= y[mid] and x[mid+1] < y[mid+1]:
            return mid
        elif (x[mid] > y[mid] or x[mid] < y[mid]) and x[mid+1] > y[mid+1]:
            return findCrossoverIndexHelper(x,y,mid+1,right) 
        else:
            return findCrossoverIndexHelper(x,y,left,mid-1)

Answered By: Santhosh E

Finding "crossover" index with modified binary search algorithm in Python

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