dsa-competitive-programming/leetcode/lc1888/SlidingWindow.cpp

class Solution {
public:
    int minFlips(string s) {
        int n = s.size();
        string t = s + s;

        int ans = INT_MAX;
        int diff1=0, diff2 = 0;
        for(int i =0;i<t.size();i++){
            char expected1 = (i%2 ? '1':'0');
            char expected2 = (i%2 ? '0':'1');

            if(t[i]!=expected1) diff1++;
            if(t[i]!=expected2) diff2++;

            if(i>=n){
                char old = t[i-n];
                char oldExpected1 = ((i-n)%2 ? '1' : '0');
                char oldExpected2 = ((i-n)%2 ? '0' : '1');

                if(old!=oldExpected1) diff1--;
                if(old!=oldExpected2) diff2--;

             
            }
               if(i>=n-1)
                    ans  = min(ans, min(diff1,diff2));
        
        }
        return ans;
    }
};

//SC : o(n)  TC : o(n)


/*
Approach:
Since the string can be rotated, we simulate all possible rotations by
doubling the string (t = s + s). Any rotation of s will appear as a
substring of length n in this doubled string. We then slide a window of
size n across t and compute how many flips are required to make the
current window alternating.

There are only two possible alternating patterns: "010101..." and
"101010...". We maintain two mismatch counters (diff1 and diff2) that
track how many characters in the current window differ from each pattern.
As the window expands, we add the mismatch contribution of the new
character. When the window exceeds size n, we remove the contribution of
the character that leaves the window.

For every window of size n, the minimum of diff1 and diff2 gives the
number of flips required for that rotation. We keep track of the global
minimum across all windows.

*/