思路

代码

class Solution {
    public int numDecodings(String s) {
        int n = s.length();
        s = ' ' + s;
        int[] f = new int[n+1];
        f[0] = 1;

        for(int i = 1; i <= n ; i++){
            int a = s.charAt(i) - '0';
            if(a >= 1 && a <= 9) f[i] += f[i-1];

            int tmp = (s.charAt(i-1)-'0') * 10 + a;
            if(i > 1 && tmp >= 10 && tmp <= 26) f[i] += f[i-2];
        }
        return f[n];
    }
}