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    Question

    In a common backtracking approach to generate

    permutations of a string, elements are swapped to explore different arrangements. Complete the line that swaps characters s[index] and s[i] def recurPermute(index, s, ans):     if index == len(s):         ans.append("".join(s))         return     for i in range(index, len(s)):         s[index], s[i] = _________ # Line to complete (swap)         recurPermute(index + 1, s, ans)         s[index], s[i] = _________ # Line to complete (backtrack swap)
    A s[i], s[index] Correct Answer Incorrect Answer
    B s[index], s[index] Correct Answer Incorrect Answer
    C s[i], s[i] Correct Answer Incorrect Answer
    D s[index+1], s[i] Correct Answer Incorrect Answer
    E s[i], s[index+1] Correct Answer Incorrect Answer

    Solution

    • Concept: Generating permutations using backtracking often involves swapping elements to explore different arrangements. After a recursive call returns, the swap needs to be undone (backtracked) to restore the array to its previous state for other branches of the recursion. • Code Analysis: o The for loop iterates from index to len(s)-1. o Inside the loop, s[index] is swapped with s[i] to place a different character at the current index position. o After the recursive call recurPermute(index + 1, s, ans), the swap needs to be reversed. • Explanation of Correct Answer (A): s[i], s[index] (for both lines) o The Pythonic way to swap two variables a and b is a, b = b, a. o To swap s[index] and s[i], the correct syntax is s[index], s[i] = s[i], s[index]. This line is used both for the initial swap before the recursive call and for the backtracking swap after the recursive call to restore the array.

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