LeetCode Decode Ways(动态规划)

题意：编码方式为a->1,...z->26，给出编码后的数字字符串，问有多种编码方式

思路：用dp(i)表示从第1个字符到第i个字符时的编码方式，因为编码一个字符时，只能是0-9,编码两个字符时，只能为10-26

所以有dp(i) = dp(i-1) + dp(i-2) ,其中如果第i-1个字符不是0-9之间的数，则dp(i-1)=0,如果i-2到i之间的字符不是10-26,则dp(i-2)=0

代码如下：

public class Solution
{public int numDecodings(String s){if (s.length() == 0) return 0;int[] dp = new int[s.length() + 1];dp[0] = 1;for (int i = 1; i <= s.length(); i++) {if (i - 1 >= 0) {int num = Integer.parseInt(s.substring(i - 1, i));if (num >= 1 && num <= 9) dp[i] += dp[i - 1];}if (i - 2 >= 0) {int num = Integer.parseInt(s.substring(i - 2, i));if (num >= 10 && num <= 26) dp[i] += dp[i - 2];}}return dp[s.length()];}
}

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