The shortest common supersequence (SCS) of two sequences X and Y
is the shortest sequence which has X and Y as subsequences.
In other words assume we’re given two strings str1 and str2, find the shortest string that has both str1 and str2 as subsequences.
This is a problem closely related to the longest common subsequence problem.
Input: str1 = "geek", str2 = "eke"
Output: "geeke"
Input: str1 = "AGGTAB", str2 = "GXTXAYB"
Output: "AGXGTXAYB"