Create Interleaving String

DP problem, dp[i][j] presents s1[0..i - 1], s2[0...j - 1] match to s3[0 ... i + j - 1]. state transition: dp[i][j] = dp[i - 1][j] && s1[i - 1] == s3[i + j - 1] || dp[i][j - 1] && s2[j - 1] == s3[i + j - 1]