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.
https://oj.leetcode.com/problems/decode-ways/
思路1：dfs。
思路2：dp。类似斐波那契数列，dp[i]表示从头到i位共有多少中组合。
如果i-1位不等于0，则dp[i]需要加上dp[i-1]的情况。
如果i-2位到i-1位在1-26之间，则dp[i]需要加上dp[i-2]情况。
public class Solution { // be careful of "0" "01" "100" public int numDecodings(String s) { if (s.length() == 0) return 0; int[] dp = new int[s.length() + 1]; dp[0] = 1; if (s.charAt(0) != ‘0‘) dp[1] = 1; else dp[1] = 0; for (int i = 1; i < s.length(); i++) { if (s.charAt(i) != ‘0‘) dp[i + 1] = dp[i]; if (s.charAt(i - 1) != ‘0‘ && Integer.parseInt(s.substring(i - 1, i + 1)) <= 26) dp[i + 1] += dp[i - 1]; } return dp[s.length()]; } public static void main(String[] args) { String s = "1010"; System.out.println(new Solution().numDecodings(s)); } }
参考：
http://blog.csdn.net/u011095253/article/details/9248109
http://blog.csdn.net/worldwindjp/article/details/19938131
[leetcode] Decode Ways,布布扣,bubuko.com
原文：http://www.cnblogs.com/jdflyfly/p/3819308.html
