Write a function:

class Solution $\{$ public String solution$($String S$,$ String T$)$; $\}$

that, given two strings S and T consisting of N and M characters, respectively, determines whether string T can be obtained from string S by at most one simple operation from the set specified below. The function should return a string:

"INSERT c$\text{'}$ if string T can be obtained from string S by inserting a single character $"$c$"$ at the beginning of the string,

"CHANGE c d if string T can be obtained from string S by replacing a single occurrence of character $"$c$"$ with a single character $"$d$"($these characters should be distinct$)$;

SWAP c d if string T can be obtained from string S by swapping two characters $"$c$"$ and $"$d$"($these characters should be distinct and in the same order as in string S$,$ exactly one swap is performed$),$

$$ NOTHING" If no operation is needed $($strings T and S are equal$)$; IMPOSSIBLE if none of the above works.