solved both

[arihantjain2198]

solved both

[super30admin]

The student's solution demonstrates a good understanding of backtracking problems, particularly in solving the Combination Sum and Expression Add Operators problems. Here are the detailed evaluations for each file:

1. CombinationSum.java:

   • Correctness: The solution correctly implements the combination sum problem using multiple approaches (helper_notTochooseFirst, helper_tochooseFirst, helper_01recursion, helper_forRecursion). All methods appear to correctly generate combinations that sum to the target.
   • Time Complexity: The time complexity is O(2^N) in the worst case, where N is the number of candidates, due to the recursive nature of the solution.
   • Space Complexity: The space complexity is O(T) where T is the target, due to the recursion stack and the path storage.
   • Code Quality: The code is well-structured but could benefit from better naming conventions for the helper methods to reflect their purpose more clearly. The use of multiple helper methods with similar functionality could be consolidated.
   • Efficiency: The solution is efficient but could be optimized by avoiding the creation of new ArrayLists in some helper methods (e.g., helper_notTochooseFirst) and using backtracking more effectively.

2. ExpressionAndOperator.java:

   • Correctness: The solution correctly implements the expression add operators problem, handling all operations (+, -, *) and avoiding invalid operands like numbers with leading zeros.
   • Time Complexity: The time complexity is O(4^N), where N is the length of the input string, due to the four choices (no op, +, -, *) at each step.
   • Space Complexity: The space complexity is O(N) due to the recursion stack and the storage of intermediate results.
   • Code Quality: The code is well-structured and readable, with clear comments explaining the logic. The use of a global variable (answer) could be avoided by passing it as a parameter.
   • Efficiency: The solution is efficient but could be optimized by reducing the number of string operations and using StringBuilder more effectively.

General Feedback:

• The student should consider adding comments to explain the purpose of each method and the overall approach.
• The use of global variables (e.g., result, answer) could be avoided by passing them as parameters or returning them from methods.
• The student should ensure consistent coding style (e.g., indentation, bracket placement) across all methods.
• Edge cases, such as empty input or target less than the smallest candidate, are handled correctly.