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$ curl repovive.com/roadmaps/google-interview-prep-v2/coding-patterns-ii-trees-graphs-dynamic-programming/lc-91-decode-ways-implementation

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$ curl repovive.com/roadmaps/google-interview-prep-v2/coding-patterns-ii-trees-graphs-dynamic-programming/lc-91-decode-ways-implementation

Repovive

LeetCode 91 Decode Ways - Implementation - Coding Patterns II: Trees, Graphs & Dynamic Programming | Google Interview Prep | Repovive

Repovive.

Google Interview Prep6 sections · 128 units7h total

Open in Course

LeetCode 91 Decode Ways - Implementation

The code

DP with two variables. At each position, check if $1$ -digit and $2$ -digit decodings are valid.

function numDecodings(s):
    if s[0] == '0':
        return 0
    n = len(s)
    prev2 = 1
    prev1 = 1
    for i from 1 to n-1:
        curr = 0
        if s[i] != '0':
            curr += prev1
        twoDigit = int(s[i-1:i+1])
        if twoDigit >= 10 and twoDigit <= 26:
            curr += prev2
        prev2 = prev1
        prev1 = curr
    return prev1

$O(n)$ time, $O(1)$ space.