[leetcode] Decode ways-优快云博客

本文解析了一道经典的算法题目——解码方式。这是一道动态规划问题，通过递推公式计算字符串的不同解码方法数量。文章提供了完整的Java实现代码。

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[url]http://leetcode.com/onlinejudge#question_91[/url]
Decode WaysJun 25 '121292 / 5011
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.

Solution: DP Problem again.


public class Solution {
    public int numDecodings(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s.length() == 0) return 0;
        int num[] = new int[s.length()];

        int n1 = Integer.parseInt(s.substring(0,1));        
        if(n1==0)  return 0;
        if(s.length() == 1) return 1;
        num[0]=1;

        int n2 = Integer.parseInt(s.substring(0,2));
        if(n2==10 || n2==20 || (n2>26 && n2%10!=0)) num[1] = 1;
        else if(n2<=26 && n2>=11) num[1] = 2;
        else return 0;

        for(int i=2;i<s.length();i++){
            n1 = Integer.parseInt(s.substring(i,i+1));
            n2 = Integer.parseInt(s.substring(i-1,i+1));
            if(n1==0 && n2==0) return 0;
            if(n1 != 0) num[i] += num[i-1];            
            if(n2>=10 && n2 <= 26) num[i] += num[i-2];
        }
        return num[s.length() -1];
    }
}