DP on prefixes: decide whether next char of s3 comes from s1 or s2.
Let dp[i][j] be whether s1[:i] and s2[:j] can form s3[:i+j].
Transition uses last char from s1 or s2.
Use 1D rolling DP over j.
DP on prefixes: decide whether next char of s3 comes from s1 or s2.
Let dp[i][j] be whether s1[:i] and s2[:j] can form s3[:i+j].
Transition uses last char from s1 or s2.
Use 1D rolling DP over j.