Basic Calculator with Addition and Subtraction

Design and implement a simple calculator that supports only addition and subtraction operations. The input is a string format mathematical expression containing only integers with addition and subtraction operations. Assume the input is always a valid expression without any extra spaces or invalid characters. Implement the corresponding calculation functionality. Example inputs: "1+2" "10-5+3" "7-2+1" The output should be the calculated result of the corresponding expression. Requirements: The time complexity should be O(n), where n is the length of the input string. Provide at least five test cases to verify the program. Example Input 3+5