leetcode Decode Ways

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Decode Ways 原题地址：

https://oj.leetcode.com/problems/decode-ways/

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.

字符串解码。

倒序解码，分析各种情况即可。注意"0"。

public class Solution {    public int numDecodings(String s) {        if (s.equals(""))return 0;int len = s.length();int[] decode = new int[len+1];decode[len] = 1;if (s.charAt(len-1) == '0')decode[len-1] = 0;elsedecode[len-1] = 1;for (int i = len-2; i >= 0; i--) {if (s.charAt(i) == '1')decode[i] = decode[i+1] + decode[i+2];else if (s.charAt(i) == '2' && s.charAt(i+1) <= '6')decode[i] = decode[i+1] + decode[i+2];else if (s.charAt(i) == '0')decode[i] = 0;elsedecode[i] = decode[i+1];}return decode[0];    }}

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