How should I practice Shortest Way to Form a Target String?

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Shortest Way to Form a Target String

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Problem statement

You may choose any subsequence of source, preserving character order, and concatenate any number of chosen subsequences.

Return the minimum number of source subsequences whose concatenation equals target. Return -1 when formation is impossible.

Function

shortestWay(source: String, target: String) → int

Examples

Example 1

source = "abc"target = "abcbc"return = 2

Use abc and then bc.

Example 2

source = "abc"target = "acdbc"return = -1

The character d never appears in the source.

Example 3

source = "xyz"target = "xzyxz"return = 3

One optimal decomposition is xz, y, and xz.

Constraints

1 <= source.length, target.length <= 100000.
Both strings contain lowercase English letters.