91 Decode Ways – Medium¶

Problem:¶

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.

Thoughts:¶

Basic dynamic programming problem. It’s very tedious with the basic and edge cases. Otherwise, it’s very straight forward.

Solutions:¶

public class Solution {
    public int numDecodings(String s) {
        //ways[i] means the number of decodes from i to s.length - 1
        if (s == null || s.length() == 0) {
            return 0;
        }
        int[] ways = new int[s.length() + 1];
        ways[s.length()] = 1;
        ways[s.length() - 1] = s.charAt(s.length() - 1) > '0'?1:0;
        for (int i = s.length() - 2; i >=0; i --) {
            if (s.charAt(i) == '0') {
                ways[i] = 0;
            }
            else {
                int tmp = (s.charAt(i) - '0') * 10 + (s.charAt(i+1) - '0');
                if (tmp > 26) {
                    ways[i] = ways[i + 1];
                }
                else {
                    ways[i] = ways[i + 1] + ways[i + 2];
                }
            }
        }
        return ways[0];
    }
}