[LeetCode] Decode Ways

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 an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

Solution: Dynamic Programming. 

Note that if a message starts with 0, there is no feasible way to decode it.

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public class Solution { private int[] ways; private int numDecodings(String s, int i) { if (i >= s.length()) { return 1; } if (s.charAt(i) == '0') { return 0; } if (ways[i] > -1) { return ways[i]; } ways[i] = numDecodings(s, i + 1); if ( i + 2 <= s.length() && Integer.valueOf(s.substring(i, i + 2)) <= 26 ) { ways[i] += numDecodings(s, i + 2); } return ways[i]; } public int numDecodings(String s) { if (s.length() == 0) { return 0; } ways = new int[s.length()]; Arrays.fill(ways, -1); return numDecodings(s, 0); } }