How should I practice Decode Ways?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Bloomberg LP onsite interview problems in FastPrep.

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Decode Ways

Bloomberg LP● MediumNEW GRADONSITE INTERVIEW

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Problem statement

The mapping 1 -> A, 2 -> B, ..., 26 -> Z encodes letters as decimal numbers.

Given a nonempty digit string digits, return the number of distinct ways to decode the entire string. A code cannot have a leading zero, so 06 is invalid while 6 is valid.

Function

numDecodings(digits: String) → int

Examples

Example 1

digits = "12"return = 2

The two decodings are AB using 1,2 and L using 12.

Example 2

digits = "226"return = 3

The valid decodings correspond to 2,2,6, 22,6, and 2,26.

Example 3

digits = "06"return = 0

A code cannot begin with 0, so the entire string has no valid decoding.

Constraints

1 <= digits.length <= 100
digits contains only characters 0 through 9.
The answer fits in a signed 32-bit integer.