97. Interleaving String - LeetCode Solution

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
        dp = []
        
        if len(s1) + len(s2) != len(s3):
            return False
        

        for i in range(len(s1)+1):
            a = []

            for j in range(len(s2)+1):
                a.append(-1)
            dp.append(a)


        def trav(s1, s2, s3, ptr1, ptr2, ptr3):
            if ptr3 == len(s3):
                return True

            if dp[ptr1][ptr2] != -1:
                return dp[ptr1][ptr2]


            a =b= False
            if ptr1 != len(s1) and s1[ptr1] == s3[ptr3]:
                a = trav(s1, s2, s3, ptr1+1, ptr2, ptr3 + 1)
            if ptr2 != len(s2)  and s2[ptr2] == s3[ptr3]:
                b = trav(s1, s2, s3, ptr1, ptr2 + 1, ptr3 + 1)

            dp[ptr1][ptr2] = a or b
            return dp[ptr1][ptr2]


        return trav(s1, s2, s3, 0, 0, 0)