LeetCode/java/decode-ways.java

class Solution {
    private static final int EMPTY = -1;

    private static int charToDigit(String s, int i) {
        return s.charAt(i) - '0';
    }

    private int numDecodings(String s, int[] dp, int i) {
        // out of bounds
        if (i < 0 || i > s.length()) {
            return 0;
        }

        // final digit
        if (i == s.length()) {
            return 1;
        }

        // starting with 0, not allowed
        if (s.charAt(i) == '0') {
            return 0;
        }

        // solution has been already precomputed
        if (dp[i] != EMPTY) {
            return dp[i];
        }

        // taking just one digit
        int result = numDecodings(s, dp, i + 1);

        // taking two digits
        if (i + 1 < s.length() && charToDigit(s, i) * 10 + charToDigit(s, i + 1) <= 26) {
            result += numDecodings(s, dp, i + 2);
        }

        // save and return
        dp[i] = result;
        return result;
    }

    public int numDecodings(String s) {
        int[] dp = new int[s.length()];
        for (int i = 0; i < dp.length; ++i) {
            dp[i] = EMPTY;
        }

        return numDecodings(s, dp, 0);
    }
}