LeetCode 1092 Shortest Common Supersequence - The LCS Connection

The length of SCS relates directly to LCS: $|SCS| = |A| + |B| - |LCS|$ Why? The LCS appears once in the supersequence. Everything else from A and B must be added. You add $|A| - |LCS|$ characters from A and $|B| - |LCS|$ characters from B. Total: $|LCS| + (|A| - |LCS|) + (|B| - |LCS|) = |A| + |B| - |LCS|$.

To construct the actual string, you need to backtrack through the LCS DP table.