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Min Swaps to Group Ones Efficiently

Aug 2, 2024

Minimum Swaps to Group All Ones Together: Part Two

Overview

  • The problem involves grouping all ones in an array (containing only zeros and ones) together using minimum swaps.
  • The challenge includes handling circular aspects of the array.

Problem Explanation

  • Given an array of zeros and ones, the goal is to group all ones together.
  • Example:
    • For an array like [0, 1, 0, 1, 1], we need to swap elements to make ones contiguous.
    • Initial swap example: swap [0, 1] to get [1, 1, 1, 0, 0] (1 swap).

Approach

  1. Initial Thoughts:

    • Consider finding the minimum length window that contains all ones and ignoring circular properties initially.
    • Calculate: length of window - count of ones = number of swaps needed.
  2. Counter Example:

    • A longer string like [1, 0, 1, 0, 1, 1] with holes can lead to overestimation of swaps.
    • Actual swaps needed could be less than calculated (2 instead of 4).
  3. New Approach:

    • Find a window of size equal to the number of ones (k) that contains the maximum number of ones.
    • Example: In [1, 0, 1, 0, 1, 1] with 4 ones, a window of size 4 can help determine how many zeros need to be swapped.
    • Resulting swaps = k - max number of ones in the window.

Handling Circularity

Method 1: Concatenation

  • Create a new concatenated array (original + original) to handle circularity.
  • This way, we can treat the end and beginning of the array as one continuous segment.
  • Use sliding window technique to find the maximum number of ones in any valid subarray.

Method 2: Constant Space

  • Instead of concatenating the array, use two pointers for a sliding window approach.
  • Right pointer goes until 2*n (where n is the original array length), using modulo to avoid out-of-bounds errors.
  • Count current window ones and keep track of the maximum found window.*

Sliding Window Implementation

  • Initialize pointers and count total ones in the input.
  • Move the right pointer and increment window ones count if a one is found.
  • If the window size exceeds total ones, shift the left pointer and adjust the count accordingly.
    • window size = right - left + 1
    • Adjust the count of ones in the current window.

Conclusion

  • The proposed solution is efficient (linear time complexity).
  • There are two ways to handle circularity (extra space vs. constant space).
  • Final result will be the number of swaps needed to group all ones together.