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.
Given encoded message 12, it could be decoded as AB (1 2) or L (12).
The number of ways decoding 12 is 2.
分析：
用total[i]表示从i 到 string.length() - 1总共的解法。 我们可以从最后一个数字开始，最后一个数字如果在1-9之间，只有一种decode way, total[length - 1] = 1. 对于倒数第二个数字呢，如果倒数第二个数字不是0，很明显，我们至少有total[i] = total[i + 1], 如果俩个数字合在一起可以组成一个valid number, 那么total[i] += total[i + 2].
1 public class Solution { 2 /** 3 * @param s a string, encoded message 4 * @return an integer, the number of ways decoding 5 */ 6 public int numDecodings(String s) { 7 if (s == null || s.length() < 1) return 0; 8 int[] total = new int[s.length()]; 9 10 for (int i = s.length() - 1; i >= 0; i--) { 11 char c = s.charAt(i); 12 if (c >= ‘1‘ && c <= ‘9‘) { 13 if(i == s.length() - 1) { 14 total[i] = 1; 15 } else { 16 total[i] = total[i + 1]; 17 String strNumber = s.substring(i, i + 2); 18 int number = Integer.parseInt(strNumber); 19 if (number <= 26) { 20 if (i + 2 == total.length) { 21 total[i] += 1; 22 } else { 23 total[i] += total[i + 2]; 24 } 25 } 26 } 27 } 28 } 29 return total[0]; 30 } 31 }
原文：http://www.cnblogs.com/beiyeqingteng/p/5662201.html