1. Leading zero
No letter maps to a standalone leading zero.
if (s[0] === "0") return 0Typorithm · medium
Decode Ways
▷ Interactive algorithm labdecode-ways-dpdynamic-programmingstringcountingspace-optimized
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Step 1 of 8 · intro card
Learn Decode Ways
Dynamic Programming
Decode Ways counts valid letter decodings using one-digit and two-digit transitions.
Zeros are only valid inside 10 or 20.
A valid single digit uses the previous count.
A valid two-digit number adds the count from two positions back.
Preparing algorithm lab…
01
State: ways up to current index.
02
Single char contributes if not 0.
03
Two chars contribute if 10..26.
04
Use rolling two counts.
function numDecodings(s) {
if (s[0] === "0") return 0;
let twoBack = 1;
let oneBack = 1;
for (let i = 1; i < s.length; i++) {
let current = s[i] !== "0" ? oneBack : 0;
const twoDigit = Number(s.slice(i - 1, i + 1));
if (twoDigit >= 10 && twoDigit <= 26) current += twoBack;
twoBack = oneBack;
oneBack = current;
}
return oneBack;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Single digit
A non-zero current char can stand alone.
let current = s[i] !== "0" ? oneBack : 03. Two-digit code
Check whether the previous and current chars form 10..26.
const twoDigit = Number(s.slice(i - 1, i + 1))4. Valid pair
A valid pair adds ways from two positions back.
if (twoDigit >= 10 && twoDigit <= 26) current += twoBackA complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.