91. Decode Ways

91. Decode Ways

• 题目描述：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.

• 题目大意：给定一个已编码的字符串，找出可以有多少种解码方式

• 思路：DP，dp[i]表示第i个字符有多少解码方式。

• 代码

  package String;/*** @Author OovEver* @Date 2017/12/11 21:03*/public class LeetCode91 {  public static int numDecodings(String s) {      if (s == null || s.length() == 0) {          return 0;      }      int n = s.length();//        dp表示第i个元素有几种decode方式      int[] dp = new int[n + 1];//        空元素有一种解压方式      dp[0] = 1;//        第一个元素的解压方式      dp[1] = s.charAt(0) == '0' ? 0 : 1;      for(int i=2;i<=n;i++) {          int first = Integer.parseInt(s.substring(i - 1, i));          int second = Integer.parseInt(s.substring(i - 2, i));          if (first >= 1 && first <= 9) {              dp[i] += dp[i - 1];          }          if (second >= 10 && second <= 26) {              dp[i] += dp[i - 2];          }      }      return dp[n];  }}