#0280

Decode Ways

MediumDSA28 min20 XP

Problem

Digits map 1 -> A through 26 -> Z. Return the number of valid decodings for s.

Why This Matters

Decode Ways is a compact DP problem where invalid zero handling matters more than the recurrence itself.

Function Signature

def num_decodings(s):

Examples

Example 1

Inputs = "226"

Output3

226 can decode as 2-2-6, 22-6, or 2-26.

Constraints

Return the exact requested value.
Handle edge cases cleanly.
Use the intended DSA pattern when brute force would scale poorly.

CodePython

def num_decodings(s):
    """
    Input example: s = "226"
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

def num_decodings(s):
    """
    Input example: s = "226"
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Three decodings

Input"226"

Expected3

There are three valid splits.

Leading zero

Input"06"

Expected0

No code starts with zero.

Hidden Test Categories

string with 10string with 30long repeated ones