Leetcode Decode Ways

A message containing letters from was A-Z 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 is "12" 2.

Problem Solving Ideas:

The typical dynamic programming

Judging dp[i-1], dp[i-2] is valid

Recurrence Formula:dp[i] + = Dp[i-1] if (s (i-1, i) is valid (1-9)) + Dp[i-2] (if s (i-2, i)) are valid (10-26))

Base Case:dp[0] =1, dp[1] = 1 (s (0) is valid) or 0 (s (0) are not valid)

Java Code:

 Public classSolution { Public intnumdecodings (String s) {if(s = =NULL|| S.length () = = 0 | | S.equals ("0")){            return0; }        int[] DP =New int[S.length () +1]; //base case:dp[0] dp[1]Dp[0] = 1; if(IsValid (s.substring (0,1)) {dp[1] = 1; }Else{dp[1] = 0; }         for(inti = 2; I<= s.length (); i++){            if(IsValid (S.substring (i-1, i))) {Dp[i]+ = Dp[i-1]; }            if(IsValid (S.substring (i-2, i))) {Dp[i]+ = Dp[i-2]; }        }        returndp[s.length ()]; }         Public BooleanIsValid (String s) {if(S.charat (0) = = ' 0 ') {            return false; }        intValue = Integer.parseint (s);//Integer.parseInt:translate string to int        returnValue >=1 && value <= 26; }}

Reference:

1. http://www.programcreek.com/2014/06/leetcode-decode-ways-java/

2. http://www.cnblogs.com/springfor/p/3896162.html

Leetcode Decode Ways