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Grokking the Coding Interview Patterns in C++

Solution: Permutations

Explore how to solve permutations problems in coding interviews by using a depth-first search method to recursively swap characters and generate all possible arrangement combinations. Understand the underlying subset pattern and analyze the time and space complexity for efficient implementation.

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Statement

Given an input string, word, return all possible permutations of the string.

Note: The order of permutations does not matter.

Constraints:

  • All characters in word are unique.

  • 1≤1 \leq word.length ≤6\leq 6

  • All characters in word are lowercase English letters.

Pattern: Subsets

Problems such as this one, where we need to find the combinations or permutations of a given string, are good examples to solve using the subsets pattern as it describes an efficient Depth-First Search (DFS) approach to handle all these problems.

Solution

Let’s discuss a few basics first. We know that n!n! is the number of permutations for a set of size nn. Another obvious and important concept is that if we choose an element for the first position, then the total permutations of the remaining elements are (n−1)!(n-1)!.

For example, if we’re given the string “abcd” and we pick “a” as our first element, then for ...