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.
用DP算法来解比较简单,需维护一个num[s.length()+1]数组,用来存储前i位子串的不同方式的编码数,最后返回num[s.length()]即可
public class Solution { public int numDecodings(String s) { if(s.length()==0||s.contains("00")||(s.length()>0&&s.charAt(0)=='0')) return 0; int[] num = new int[s.length()+1]; num[0] = 1; num[1] = 1; for(int i=1;i<s.length();i++) { int tmp = Integer.valueOf(s.substring(i-1,i+1)); if(s.charAt(i)=='0') { if(tmp>=30) return 0; else { num[i] = num[i-1]; num[i+1] = num[i]; } } else { if(tmp>26||tmp<10) num[i+1] = num[i]; else num[i+1] = num[i]+num[i-1]; } } return num[s.length()]; } }