Balanced Bit Window

Given a string of length N consisting of '0', '1', and '?', and two integers K and C, replace every '?' with either '0' or '1' such that in every sliding window of size K, the number of '1's is exactly C. Print the lexicographically smallest valid string, or 'None' if no such string exists. Example: Input: 4 2 1 ??1? Output: 1010 (Explanation: Windows of size 2 are (1,0) and (0,1) and (1,0). All have one '1'.)