Recursion & DP Target: 15s
2D DP checks if characters from s1 and s2 interleave to form s3.
dp[0][0] = True
for i in range(m+1):
for j in range(n+1):
if i and s1[i-1] == s3[i+j-1]: dp[i][j] |= dp[i-1][j]
if j and s2[j-1] == s3[i+j-1]: dp[i][j] |= dp[i][j-1]Type it from memory. Go.