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.
Analysis:
This is a DP problem. d[i] is the number of ways to decode the first i chars.
d[i]=d[i-2]+d[i-1] if num(i-1,i) is a valid number and num(i,i) is a valid number.
NOTE: for a string with 2 chars, the number should be between 10 and 26. For a string with 1 char, the valid number should between 1 and 9. "0","01","02"...like this are all invalid.
Solution:
1 public class Solution { 2 public int numDecodings(String s) { 3 if (s.length()==0) return 0; 4 int[] d = new int[s.length()+1]; 5 d[0]=1; 6 for (int i=1;i<d.length;i++){ 7 d[i]=0; 8 int start = 0; 9 if (i-2>start) start = i-2; 10 for (int k=start;k<i;k++){ 11 String temp = s.substring(k,i); 12 if (temp.length()==1&&!temp.equals("0")) 13 d[i]+=d[k]; 14 if (temp.length()==2){ 15 int num = Integer.parseInt(temp); 16 if (num<=26 && num>=10) 17 d[i]+=d[k]; 18 } 19 } 20 } 21 return d[s.length()]; 22 } 23 }
原文：http://www.cnblogs.com/lishiblog/p/4098786.html