DecodeWays.java

/*
 Author:     King, wangjingui@outlook.com
 Date:       Dec 25, 2014
 Problem:    Decode Ways
 Difficulty: Easy
 Source:     https://oj.leetcode.com/problems/decode-ways/
 Notes:
 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: 1. dp. Time : O(n); Space : O(1).
 */

public class Solution {
    public int numDecodings(String s) {
        if (s.length() == 0 || s.charAt(0) == '0') return 0;
        int N = s.length();
        int f0 = 1, f1 = 1;
        for (int i = 1; i < N; ++i) {
            if (s.charAt(i) == '0') f1 = 0;
            int num = s.charAt(i) - '0' + (s.charAt(i-1) - '0') * 10;
            if (num < 10 || num > 26) {
                f0 = 0;
            }
            int tmp = f1;
            f1 = f1 + f0;
            f0 = tmp;
        }
        return f1;
    }
}