LeetCode 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.

思路：DP，设dp[i]表示长度为i时的可能，那么dp[i] = dp[i-1]，如果s[i-1]非零的话；同理只要s[i-2]!=‘0‘且在[1,26]的话，那么这个两位数也是一种可能，所以dp[i]+=dp[i-2].

public class Solution {
    public int numDecodings(String s) {
    	if (s == null || s.length() == 0) return 0;
    	if (s.charAt(0) == '0') return 0;
    	
    	int dp[] = new int[s.length()+1];
    	dp[0] = 1;
    	dp[1] = 1;
    	for (int i = 2; i <= s.length(); i++) {
    		int tmp = Integer.parseInt(s.substring(i-1, i));
    		if (tmp != 0) dp[i] = dp[i-1];
    		if (s.charAt(i-2) != '0') {
    			tmp = Integer.parseInt(s.substring(i-2, i));
    			if (tmp >= 1 && tmp <= 26) 
    				dp[i] += dp[i-2];
    		}
    	}
    	
    	return dp[s.length()];
    }
}

LeetCode Decode Ways

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原文地址：http://blog.csdn.net/u011345136/article/details/45066531