Minimum Swaps to Match Strings | Dataford Interview Questions

A PhonePe mobile flow receives two representations of the same token with characters in different positions. Given two strings, compute the minimum number of arbitrary swaps of characters in the first string required to transform it into the second.

A swap may exchange characters at any two positions. Return -1 if the strings cannot be made equal. To make the mapping unambiguous, every character appears at most once in each string.

Formal Specification

Implement minimum_swaps(source, target):

Input: Two strings source and target containing distinct lowercase English letters.
Output: An integer representing the minimum number of arbitrary swaps needed, or -1 if transformation is impossible.
The strings are considered equal only when their characters match at every position.

Constraints

1 <= len(source), len(target) <= 10^5
Both strings contain only lowercase English letters
Each character appears at most once in each string
A valid transformation requires equal lengths and equal character sets

Function Signature

def minimum_swaps(source, target):

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