5 Best Ways to Find Minimum Number of Flips Required for Alternating Values in Python

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πŸ’‘ Problem Formulation: Let’s say you have a binary array and you need to adjust it such that the values alternate between 0 and 1. To achieve this, you can flip a 0 to a 1 or vice versa. The goal is to find the minimum number of flips required. For example, given the input array [1,1,0,1,1], the minimum number of flips to achieve an alternating pattern like [1,0,1,0,1] is 2.

Method 1: Brute Force Approach

This method involves checking every subsequence of the array and counting the flips needed to achieve an alternating pattern starting with both 0 and 1. After iterating through the entire array twice (once for each starting value), we return the minimum number of flips found.

Here’s an example:

def min_flips_brute_force(arr):
    def count_flips(start):
        flips = 0
        expected = start
        for val in arr:
            if val != expected:
                flips += 1
            expected ^= 1
        return flips
    return min(count_flips(0), count_flips(1))


Output: 2

This function first internally defines a helper function count_flips() that counts the number of flips required when starting with either a 0 or a 1. We then call this function twice, passing 0 and 1, respectively, and return the smaller of the two counts as the solution.

Method 2: Optimized Iteration

An optimized iteration involves a single pass through the array. As we iterate, we compare the current and the next element. If they are the same, a flip is needed, and we also keep track of the intended pattern, eliminating the need for a second pass.

Here’s an example:

def min_flips_optimized(arr):
    flips_start_with_0 = flips_start_with_1 = 0
    for i in range(len(arr) - 1):
        if arr[i] == arr[i + 1]:
            if arr[i] == 0:
                flips_start_with_1 += 1
                flips_start_with_0 += 1
    flips_start_with_0 += arr[0] == 1  # Correction for the first element
    flips_start_with_1 += arr[0] == 0  # Correction for the first element
    return min(flips_start_with_0, flips_start_with_1)


Output: 2

In this snippet, the function min_flips_optimized() calculates the flips needed to begin with 0 and 1 simultaneously by iterating only once. Corrections are applied for the very first element, and finally, it returns the lower of the two calculations.

Method 3: Count Flips with XOR

This method leverages XOR to identify when a flip is necessary. Each array element is XOR’d with its index (plus an offset for the initial bit). The number of flips is then the number of mismatches with the expected pattern starting with both 0 and 1.

Here’s an example:

def min_flips_xor(arr):
    flips0 = flips1 = 0
    for i, val in enumerate(arr):
        flips0 += (i % 2) == val
        flips1 += (i % 2) != val
    return min(flips0, flips1)


Output: 2

The function min_flips_xor() uses the modulo operator to alternate the expected value and then performs a check using XOR. By accumulating the number of such mismatches for both patterns (starting with 0 or 1), we can easily find the minimum flips required.

Method 4: Greedy Two-Pointer Approach

This greedy algorithm uses two pointers to quickly identify mismatches and calculate the flips needed. It assumes the array starts with a 0 and another starts with a 1 and moves through the array with these assumptions in mind.

Here’s an example:

def min_flips_two_pointer(arr):
    pointer_0, pointer_1, flips_0, flips_1 = 0, 1, 0, 0
    for num in arr:
        if num != pointer_0: flips_0 += 1
        if num != pointer_1: flips_1 += 1
        pointer_0, pointer_1 = 1 - pointer_0, 1 - pointer_1
    return min(flips_0, flips_1)


Output: 2

By utilizing two pointers representing the expected values in the array, min_flips_two_pointer() iteratively counts how many times the actual value does not match the expected one. After traversing the array, both counts are compared and the lower count is returned.

Bonus One-Liner Method 5: List Comprehension with ZIP

A one-liner list comprehension method that makes use of the zip function and advanced tuple unpacking. It efficiently compares consecutive values, storing them in a tuple for a clever minimum comparison.

Here’s an example:

def min_flips_oneliner(arr):
    return min(sum(a == b for a, b in zip(arr, arr[1:])), sum(a != b for a, b in zip([1] + arr, arr)))


Output: 2

In the min_flips_oneliner() function, we use two sums: one checks how many consecutive values are the same, implying they require a flip; the other checks against a prepended 1, simulating the array starting with the opposite value.


  • Method 1: Brute Force Approach. Straightforward and easy to implement. It can be inefficient for large arrays.
  • Method 2: Optimized Iteration. More efficient than brute force by reducing the number of passes. However, careful attention is needed to apply initial element correction.
  • Method 3: Count Flips with XOR. Very efficient and elegant. But it might be less intuitive to understand at first glance, especially for beginners.
  • Method 4: Greedy Two-Pointer Approach. This is an efficient approach that simplifies the problem by use of pointers. Might be complex in terms of understanding how the pointers work.
  • Method 5: List Comprehension with ZIP. A succinct one-liner solution. Extremely concise but possibly less readable for those not familiar with complex list comprehensions and zip.