Given 2 strings s1 and s2.
Find the longest common subsequent from the given 2 strings.
ex:
"ABCDA" and "MALBQDZ" = "ABD"
"ABBTGGA" and "BMTGXA" = "BTGA"
logic:
- CHECK from S1, result1
- CHECK from S2, result2
- if result1 > resullt2, result = result1. otherwise, result = result2.