0091-decode-ways.java

// Optimal
class Solution {

    public int numDecodings(String s) {
        int twoBack = 1; // empty string
        int oneBack = s.charAt(0) == '0' ? 0 : 1;
        int current = oneBack;
        for (int i = 2; i < s.length() + 1; i++) {
            current = 0;
            if (s.charAt(i - 1) != '0') {
                current += oneBack;
            }
            if (
                s.charAt(i - 2) == '1' ||
                (s.charAt(i - 2) == '2' && s.charAt(i - 1) < '7')
            ) {
                current += twoBack;
            }
            twoBack = oneBack;
            oneBack = current;
        }
        return current;
    }
}

//bottom up
class Solution {

    public int numDecodings(String s) {
        int[] dp = new int[s.length() + 1];
        dp[0] = 1; // empty string
        dp[1] = s.charAt(0) == '0' ? 0 : 1;
        for (int i = 2; i < s.length() + 1; i++) {
            if (s.charAt(i - 1) != '0') {
                dp[i] += dp[i - 1];
            }
            if (
                s.charAt(i - 2) == '1' ||
                (s.charAt(i - 2) == '2' && s.charAt(i - 1) < '7')
            ) {
                dp[i] += dp[i - 2];
            }
        }
        return dp[s.length()];
    }
}

//top down with memoization
class Solution {

    public int numDecodings(String s) {
        return numDecodings(s, 0, new Integer[s.length()]);
    }

    private int numDecodings(String s, int i, Integer[] dp) {
        if (i == s.length()) return 1;
        if (s.charAt(i) == '0') return 0;
        if (dp[i] != null) return dp[i];
        int count = 0;
        count += numDecodings(s, i + 1, dp);
        if (
            i < s.length() - 1 &&
            (s.charAt(i) == '1' || s.charAt(i) == '2' && s.charAt(i + 1) < '7')
        ) {
            count += numDecodings(s, i + 2, dp);
        }
        dp[i] = count;
        return dp[i];
    }
}