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)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.
