Decode Ways
Count the ways to decode a string of digits where 'A' = 1 … 'Z' = 26.
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Lesson notes
Try it before you watch
Restate the problem in your own words, list the edge cases, and sketch a solution with its running time. Then play the video and compare.
Reveal the key idea
dp[i] adds dp[i − 1] when the last digit is 1–9 and dp[i − 2] when the last two digits form 10–26. Only two previous values are needed.
Pattern: Dynamic programming. Define a state, write the recurrence between states, and compute each state only once.
Complexity
| Measure | Bound |
|---|---|
| Time | O(n) |
| Extra space | O(1) |
Walkthroughs often start from a simpler approach first; aim to reach these bounds. New to Big-O? Read understanding algorithmic complexity.