there are two ways we can come to i + j in s3: s3[i+j-1] = s1[i-1] with the fact we come to i + j - 1 from s2 in last step; or we come from s1 in last step and s3[i+j-1] == s2[j-1]

there are two ways we can come to i + j in s3: s3[i+j-1] = s1[i-1] with the fact we come to i + j - 1 from s2 in last step; or we come from s1 in last step and s3[i+j-1] == s2[j-1]