The idea was to find the answer if the string was to end at ith index such that s[i] = '*'.

dp[i] represents the minimum number of replacements for the prefix s[0]....s[i] where s[i]='*'.

dp[index of first occurrence of '*' in s] = 1 (base condition).

The transition is pretty simple, if s[i]='*', and because it is the last character of our prefix, we will have to change it to 'x'.

So we traverse over all the previous indices 'j' of s such that s[j]='*' and (i — j)<=k (due to the range given) and do the transition dp[i]=min(dp[i],dp[j]+1) (+ 1 because we are replacing the current character to 'x' as well).

Our final answer will be dp[index of last '*' character in s].