1946. Largest Number After Mutating Substring 
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Problem
You are given a string num, which represents a large integer. You are also given a 0-indexed integer array change of length 10 that maps each digit 0-9 to another digit. More formally, digit d maps to digit change[d].
You may choose to mutate a single substring of num. To mutate a substring, replace each digit num[i] with the digit it maps to in change (i.e. replace num[i] with change[num[i]]).
Return a string representing the largest possible integer after mutating (or choosing not to) a single substring of num.
A substring is a contiguous sequence of characters within the string.
Example 1:
Input: num = "132", change = [9,8,5,0,3,6,4,2,6,8] Output: "832" Explanation: Replace the substring "1": - 1 maps to change[1] = 8. Thus, "132" becomes "832". "832" is the largest number that can be created, so return it.
Example 2:
Input: num = "021", change = [9,4,3,5,7,2,1,9,0,6] Output: "934" Explanation: Replace the substring "021": - 0 maps to change[0] = 9. - 2 maps to change[2] = 3. - 1 maps to change[1] = 4. Thus, "021" becomes "934". "934" is the largest number that can be created, so return it.
Example 3:
Input: num = "5", change = [1,4,7,5,3,2,5,6,9,4] Output: "5" Explanation: "5" is already the largest number that can be created, so return it.
Constraints:
1 <= num.length <= 105numconsists of only digits0-9.change.length == 100 <= change[d] <= 9