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Rotate String - Solution Explanation

This repository provides step-by-step explanations for solving the "Rotate String" problem across multiple programming languages: C++, Java, JavaScript, Python, and Go.

Problem Summary

Given two strings, s and goal, determine if s can become goal after a certain number of rotations. A rotation involves moving the leftmost character to the rightmost position, one character at a time.

For example, if s = "abcde":

  • After one rotation: "bcdea"
  • After two rotations: "cdeab"

The goal is to check if such rotations can make s equal to goal.

Approach and Solution

Step 1: Check Lengths

  • First, verify if the lengths of s and goal are equal.
  • If they are not, then s cannot be rotated to become goal, so we immediately return false.

Step 2: Concatenate s with Itself

  • Concatenate s with itself to create a new string, doubled = s + s.
  • This combined string includes all possible rotations of s as substrings. By doubling s, each possible rotation is represented as a contiguous substring within doubled.

Step 3: Check for goal in doubled

  • Use a substring search to see if goal is contained within doubled.
  • If goal is found within doubled, then s can indeed be rotated to match goal, and we return true.
  • If goal is not found in doubled, then it's impossible to match s to goal via rotations, so we return false.

Detailed Code Explanations

C++ Code Explanation

  1. Check Lengths: First, check if the length of s is equal to goal. If they are different, return false.
  2. Concatenate Strings: Create a new string by concatenating s with itself.
  3. Search for Substring: Use the find function to check if goal is a substring of the concatenated string. If goal is found, return true; otherwise, return false.

Java Code Explanation

  1. Check Lengths: First, compare the lengths of s and goal. If they don’t match, return false.
  2. Concatenate Strings: Create a new string by combining s with itself.
  3. Search for Substring: Use the contains method to see if goal is a substring of the concatenated string. If goal exists in this combined string, return true; otherwise, return false.

JavaScript Code Explanation

  1. Check Lengths: Begin by comparing the lengths of s and goal. If they are different, immediately return false.
  2. Concatenate Strings: Construct a new string by concatenating s with itself.
  3. Search for Substring: Use the includes method to determine if goal is a substring of the concatenated string. If goal is found, return true; otherwise, return false.

Python Code Explanation

  1. Check Lengths: Start by checking if the length of s matches the length of goal. If not, return false.
  2. Concatenate Strings: Create a new string by concatenating s with itself.
  3. Search for Substring: Use the in keyword to check if goal is a substring of the concatenated string. If goal is found within, return true; otherwise, return false.

Go Code Explanation

  1. Check Lengths: First, verify if s and goal have the same length. If they don’t, return false.
  2. Concatenate Strings: Create a new string by concatenating s with itself.
  3. Search for Substring: Use strings.Contains to check if goal is within the concatenated string. If goal is a substring, return true; otherwise, return false.

Complexity Analysis

  • Time Complexity: (O(n)), where (n) is the length of s or goal, because checking if goal is a substring of the doubled string takes linear time.
  • Space Complexity: (O(n)), as we create a concatenated string of size (2 \times n).

Conclusion

This approach provides an efficient solution by leveraging string concatenation and substring search instead of generating each rotation explicitly. The method is consistent across different languages, offering a uniform understanding and solution to the problem.