Check out example codes for "what is lexicographically smallest string". It will help you in understanding the concepts better.

Code Example 1

Input: A = "hello", B = "world", S = "hold"
Output: "hdld"
Explanation:  Based on the equivalency information in A and B, we can group their characters as [h,w], [d,e,o], [l,r]. So only the second letter 'o' in S is changed to 'd', the answer is "hdld".

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