Decode Ways

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Problem

A message containing only digits can be decoded back into letters using the mapping 'A' → "1", …, 'Z' → "26". Given a string of digits, return the number of distinct ways it can be decoded.

Examples

Example 1

Input:s = "12"

Output:2

"AB" (1 2) or "L" (12).

Example 2

Input:s = "06"

Output:0

Leading zeros aren't valid single-digit codes.

Constraints

1 <= s.length <= 100

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How to approach it: the 1-D Dynamic Programming pattern

Break a problem into overlapping subproblems along a single dimension — like "the answer up to index i" — solve each one once, and reuse the result instead of recomputing it.

Look for this pattern when

A greedy or purely recursive approach recomputes the same subproblem repeatedly.
The problem asks for a count, a minimum/maximum, or a yes/no reachability over a sequence, where the answer at position `i` depends on answers at earlier positions.

Read the full 1-D Dynamic Programming guide →

Video walkthroughs

Original problem on LeetCode ↗