Construct Smallest Number From DI String

class Solution {
public:
    string smallestNumber(string pattern) {
        int n = pattern.length();
        string result = "";
        int stack[n + 1];
        int index = 0;

        for (int i = 0; i <= n; i++) {
            stack[index++] = i + 1;

            if (i == n || pattern[i] == 'I') {
                while (index > 0) {
                    result += to_string(stack[--index]);
                }
            }
        }

        return result;
    }
};