LeetCode 91. Decode Ways

decode-ways.java

/**
 * https://leetcode.com/problems/decode-ways/
 *
 * O(n) runtime, O(1) space
 *
 * this is a typical DP question
 * use a rolling array to get O(1) space
 */

public class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        if (s.length() == 1) {
            return s.charAt(0) != '0' ? 1 : 0;
        }
        int[] nums = new int[3];
        if (s.charAt(0) != '0') {
            nums[0] = 1;
        } else {
            return 0;
        }
        int n = (s.charAt(0) - '0') * 10 + (s.charAt(1) - '0');
        nums[1] = (s.charAt(1) != '0' ? nums[0] : 0) +
                  (10 <= n && n <= 26 ? 1 : 0);
        for (int i = 2; i < s.length(); i++) {
            n = (s.charAt(i-1) - '0') * 10 + (s.charAt(i) - '0');
            nums[i%3] = (s.charAt(i) != '0' ? nums[(i-1)%3] : 0) +
                        (10 <= n && n <= 26 ? nums[(i-2)%3] : 0);
        }
        return nums[(s.length()-1)%3];
    }
}

It is a best solution found that very popular and helpful:
https://www.youtube.com/watch?v=Z2NTdsOjvAQ&ab_channel=EricProgramming