Before any machinery: s1 = 'ab', s2 = 'cd', s3 = 'abcde'. Can s3 be an interleaving of s1 and s2?
Can these three strings even be an interleaving?
OK. Now try this one: 'aabcc' and 'dbbca' ⟶ 'aadbbcbcac'. Lengths match (5 + 5 = 10). Can it interleave? Let's find out.