Backtrack

[MeghaN28]

No description provided.

[super30admin]

Combination Sum (CombinationSumForBacktrack.java)

Excellent work! Your solution is actually more efficient than the reference solution. Here are some observations:

Strengths:

1. Optimal backtracking pattern: Using path.add() followed by path.remove(path.size() - 1) is the standard and most efficient way to implement backtracking.
2. Proper deep copy when adding to results: You correctly use new ArrayList<>(path) when adding to res to avoid reference issues.
3. Clean for-loop approach: The for loop starting from indx is the canonical way to solve this problem, allowing unlimited reuse of each candidate.
4. Good variable naming: The names are descriptive and clear.

Minor suggestions:

1. Consider adding a comment explaining the backtracking pattern for future readers.
2. The variable name indx could be index or start for better readability (though this is subjective).
3. You could add an early termination optimization: if candidates[i] > target, you could break out of the loop (assuming candidates are sorted), but this isn't strictly necessary.

Overall, your solution demonstrates a strong understanding of backtracking and is more efficient than the reference solution.

VERDICT: PASS

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Expression Add Operators (CombinationSumRecurseBacktrack.java)

1. Wrong Problem: You have submitted a solution for "Combination Sum" instead of "Expression Add Operators". Please re-read the problem carefully and implement the correct solution.

2. Understanding the Problem: For "Expression Add Operators", you need to insert +, -, or * between each digit of the input string num such that the resulting expression evaluates to target. The output should be a list of valid expression strings.

3. Key Algorithm Insight: The tricky part is handling multiplication precedence. When you multiply, you need to undo the previous addition/subtraction and apply the multiplication to the previous operand. The reference solution uses a tail variable to track the last operand for this purpose.

4. Leading Zeros: You must skip numbers with leading zeros (e.g., "05" is invalid). The reference solution handles this with if(num.charAt(pivot) == '0' && i != pivot) break;.

5. Backtracking Pattern: Use a StringBuilder with setLength() for efficient string manipulation during backtracking, as shown in the reference solution.

6. Time Complexity: The expected complexity is O(4^n) where n is the length of num, since at each position you have 4 choices (no operator, +, -, *).

VERDICT: NEEDS_IMPROVEMENT