Statistical letter decoding method Total <Data Structures and Algorithms> recursive algorithm

　　Entitled LeetCode first 91 questions, a message containing the letters AZ is encoded by:

　　'A' -> 1

　　'B' -> 2

　　…

　　'Z' -> 26

　　Given contain only a non-empty string of numbers, the total number of decoding approach.

 

　　The title can be used to resolve recursive, directly attached to the code:

 

public static int numDecodings(String s) {

    if (s.charAt(0) == '0') return 0;

    char[] chars = s.toCharArray();
    return decode(chars, chars.length - 1);
} 

// string into a character array, using a recursive function decode, forward recursively from the last character 
Private int decode ( char [] chars, int index) {
     // if the index is 0, the processing head to a string , 
    IF (index <= 0) {

　　　　　　return 1;
　　 }
　　 int count = 0;  
    char curr = chars[index];
    char prev = chars[index - 1];
   
    // current character than "0" is large, the direct use of the resulting string before it obtained      
    IF (Curr> '0' ) {
        count = decode(chars, index - 1);
    }
    
    // two numbers of characters needs to judge whether a number between 1-26 
    IF (PREV == '. 1' || (PREV == '2' && Curr <= '. 6' )) {
        count += decode(chars, index - 2);
    }
        
    return count;
}