Given a string str containing only lowercase letters, generate a string with the same letters, but in uppercase.
str = "geeks"
str = "geeksforgeeks"
You don't need to read input or print anything. Your task is to complete the function to_upper() which takes the string str as an argument and returns the resultant string.
Expected Time Complexity: O(length of the string).
Expected Auxiliary Space: O(1).
1 ≤ length of the string ≤ 50
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