1946. Largest Number After Mutating Substring

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.

class Solution {
public:
    string maximumNumber(string num, vector<int>& change, bool changed = false) {
        for(auto& c : num) {
            int op(c & 0b1111);
            if(change[op] >= (op)) {
                if(change[op] > (op)) changed = true;
                c = change[op] | 0b110000;
            } else if(changed) return num;
        }
        return num;
    }
};