995_Minimum_Number_of_K_Consecutive_Bit_Flips.cpp

/*
995. Minimum Number of K Consecutive Bit Flips
You are given a binary array nums and an integer k.
A k-bit flip is choosing a subarray of length k from nums and simultaneously changing every 0 in the subarray to 1, and every 1 in the subarray to 0.
Return the minimum number of k-bit flips required so that there is no 0 in the array. If it is not possible, return -1.
A subarray is a contiguous part of an array.
Example 1
Input: nums = [0,1,0], k = 1
Output: 2
Explanation: Flip nums[0], then flip nums[2].
Example 2:
Input: nums = [1,1,0], k = 2
Output: -1
Explanation: No matter how we flip subarrays of size 2, we cannot make the array become [1,1,1].
Example 3:
Input: nums = [0,0,0,1,0,1,1,0], k = 3
Output: 3
Explanation: 
Flip nums[0],nums[1],nums[2]: nums becomes [1,1,1,1,0,1,1,0]
Flip nums[4],nums[5],nums[6]: nums becomes [1,1,1,1,1,0,0,0]
Flip nums[5],nums[6],nums[7]: nums becomes [1,1,1,1,1,1,1,1]
Constraints:
1 <= nums.length <= 105
1 <= k <= nums.length
*/
class Solution {
public:
    int minKBitFlips(vector<int>& nums, int k) {
        int ans = 0, flips = 0;
        for(int i = 0; i < nums.size(); i++) {
            if((nums[i] + flips) % 2 == 0) {
                if(i > nums.size() - k) {
                    return -1;
                } else {
                    ans++;
                    flips++;
                    nums[i] = -1;
                }
            }
            if(i + 1 >= k) {
                flips -= (nums[i - k + 1] < 0);     
            }
        }
        return ans;
    }
};