91. Decode Ways

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LeetCode

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Problem

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

‘A’ -> 1 ‘B’ -> 2 … ‘Z’ -> 26 Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6)..

Code

class Solution {
    public int numDecodings(String s) {
        if(s == null || s.length() == 0 || s.charAt(0) == '0') return 0;
        int[] dp = new int[s.length() + 1];
        dp[0] = 1;
        dp[1] = 1;

        for(int i = 1; i < s.length(); i++){
            // "101"
            if(s.charAt(i) != '0'){
                dp[i + 1] += dp[i];
            }

            int num = Integer.valueOf(s.substring(i - 1, i + 1));
            if(num >= 10 && num <= 26){
                dp[i + 1] += dp[i - 1];
            }
        }

        return dp[s.length()];
    }
}