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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