91. Decode Ways

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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.

DP思路，dp[i]与dp[i-1]，dp[i-2]都有关系，参考http://www.cnblogs.com/ganganloveu/p/3739606.html

public class Solution {    public int numDecodings(String s) {    if("".equals(s))return 0;    if("0".equals(s))return 0;    if(s.length() == 1)return 1;        if(s.charAt(0) == '0')return 0;        char[] cs = s.toCharArray();    int[] dp = new int[s.length() + 1];    dp[0] = 1;dp[1] = 1;        for(int i=1; i<s.length(); i++) {    if(cs[i-1] == '0') {    if(cs[i] == '0')    return 0;    else    dp[i+1] = dp[i];    } else if(cs[i-1] == '1') {    if(cs[i] == '0')    dp[i+1] = dp[i-1];    else    dp[i+1] = dp[i] + dp[i-1]; // 因为cs[i-1]!=0，所以一定可以单枪匹马match    } else if(cs[i-1] == '2') {    if(cs[i] == '0')    dp[i+1] = dp[i-1];    else if(cs[i]>='1' && cs[i]<='6')    dp[i+1] = dp[i] + dp[i-1];    else    dp[i+1] = dp[i];    } else {    if(cs[i] == '0')    return 0;    else    dp[i+1] = dp[i];    }    }        return dp[s.length()];    }}

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