91. Decode Ways（js）

91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
题意：给定一个数字字符串，数字对应字符所处的位置所有情况个数
代码如下：

/**
 * @param {string} s
 * @return {number}
 */
var numDecodings = function(s) {
    let n=s.length
        if(n==0) return 0;
        let count=new Array(n+1).fill(0);
        count[n]=1;
        count[n-1]=s.charAt(n-1)=='0'?0:1;
        
        for(let i=n-2;i>=0;i--){
            if(s.charAt(i)=='0') continue;
            else{
                count[i]=(s.substring(i,i+2)<=26)?count[i+1]+count[i+2]:count[i+1];
            }
        }
        return count[0];
};