LeetCode – Decode Ways (Java)

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A message containing letters from A-Z is being encoded to numbers using the following mapping:

‘A’ -> 1
‘B’ -> 2
…
‘Z’ -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

Java Solution

This problem can be solve by using dynamic programming. It is similar to the problem of counting ways of climbing stairs. The relation is dp[n]=dp[n-1]+dp[n-2]. There are some cases to

public int numDecodings(String s) {
    int[] dp = new int[s.length() + 1];
    dp[0] = 1;
    if (s.charAt(0) == '0') {
        return 0;
    }
    dp[1] = 1;
 
    for (int i = 1; i < s.length(); i++) {
        char c = s.charAt(i);
        char p = s.charAt(i - 1);
 
        if (c == '0' && (p == '0' || p > '2')) {
            return 0;
        }
 
        if (p == '0') {
            dp[i + 1] = dp[i];
        } else if (p == '1') {
            if (c == '0') {
                dp[i + 1] = dp[i - 1];
            } else {
                dp[i + 1] = dp[i - 1] + dp[i];
            }
        } else if (p == '2') {
            if (c == '0') {
                dp[i + 1] = dp[i - 1];
            } else if (c <= '6') {
                dp[i + 1] = dp[i] + dp[i - 1];
            } else {
                dp[i + 1] = dp[i];
            }
        } else {
            dp[i + 1] = dp[i];
        }
 
    }
 
    return dp[s.length()];
}

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