The stack-based structure clicked pretty fast for me.
Use a stack to parse the RPN expression, building expression trees for operands and operators. Then perform an inorder traversal of the tree, adding parentheses only when the child operator has lower precedence than the parent, or when associativity requires it (e.g., right child with same precedence for left-associative operators).
Pro tip: Clarify the operator precedence and associativity rules upfront, as they determine where parentheses are needed. Also, consider edge cases like unary minus or invalid expressions to show thoroughness.
Split the input string by commas and iterate through tokens. Use a stack to build an expression tree: push operands as leaf nodes; for each operator, pop two nodes, create a new node with the operator and the two nodes as children, and push it back.
Establish a precedence map for operators (e.g., +,-: 1; *,/: 2) and note that most binary operators are left-associative. This will guide parentheses insertion during traversal.
Perform an inorder traversal of the expression tree. For each operator node, recursively generate the left and right subexpressions. Add parentheses around a child if its operator has lower precedence than the parent, or if it has equal precedence but is the right child of a left-associative operator.
Consider unary operators (e.g., negative numbers) and ensure the output is a valid infix expression. Optionally, verify by converting back to RPN or evaluating both expressions to confirm equivalence.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.