91 Decode Ways.js

// 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.

/**
 * @param {string} s
 * @return {number}
 */

var numDecodings = function(s) {
    if(!s || s[0] === '0') {
        return 0;
    }
   
    var nums = [1, 1];
    
    for(var i = 2; i <= s.length; i++) {
        var tmp;
        
        tmp = parseInt(s.substring(i - 1, i));
        
        if(tmp === 0) {
            nums[i] = 0;
        } else {
            nums[i] = nums[i-1];
        }
        
        if(s[i - 2] !== '0') {
            tmp = parseInt(s.substring(i - 2, i));
            
            if(0 < tmp && tmp <= 26) {

                nums[i] += nums[i - 2];
            }
        }
    }
    
    return nums[s.length];
};

console.log(numDecodings('10'));

// Using recusion
// It also store all the possible combination
// var numDecodings = function(s) {
//     var result = [];
    
//     function traverse(s, beg, end, cur) {
//         if(end > s.length) {
//           return 
//         }
    
//         var str = s.substring(beg, end);
//         var num = parseInt(str);
        
//         if(isNaN(num) || num === 0 || num > 26) {
//           return;
//         }
      
//         cur.push(str);
      
//         if(end === s.length){
//           result.push(cur.slice());
//           return;
//         }
      
//         traverse(s, end, end + 1, cur);
//         traverse(s, end, end + 2, cur);
//         cur.pop();
//     }
  
//     traverse(s, 0, 1, []);
//     traverse(s, 0, 2, []);
  
//     return result.length;
// };