Please solve this problem with JAVA and explain. (Do not copy the solution from online sources.)

91. Decode Ways

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.

The answer is guaranteed to fit in a 32-bit integer.

Example 1:

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

Example 2:

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

Example 3:

Input: s = "0" Output: 0 Explanation: There is no character that is mapped to a number starting with '0'. We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'. 

Example 4:

Input: s = "1" Output: 1 

Constraints:

• 1 <= s.length <= 100
• s contains only digits and may contain leading zero(s).

class Solution { public int numDecodings(String s) { } }