91. Decode Ways - medium

这道题，印象深刻，和小白讨论过。
方法是DP，关键点是“0”；
DP问题，从前往后还是从后往前？（我还是很烂的)
一般都是从后往前，个别情况例外。

自己写出来的code：

public class Solution {
    public int numDecodings(String s) {
        int len = s.length();
        if (len==0)  return 0;
        int[] mem = new int[len+1];
        mem[len] = 1;
        mem[len-1] = (s.charAt(len-1)=='0')? 0 : 1;
        for (int i=len-2; i>=0; i--) {
            if(s.charAt(i) == '0')  mem[i] = 0;
            else    
                mem[i] = (Integer.parseInt(s.substring(i, i+2)) <= 26)? mem[i+1]+mem[i+2] : mem[i+1];
        }
        return mem[0];
    }
}

答案代码和我自己写的竟然如此的像！！！！
长舒一口气。

public class Solution {
    public int numDecodings(String s) {
        int n = s.length();
        if (n == 0) return 0;
        
        int[] memo = new int[n+1];
        memo[n]  = 1;
        memo[n-1] = s.charAt(n-1) != '0' ? 1 : 0;
        
        for (int i = n - 2; i >= 0; i--)
            if (s.charAt(i) == '0') continue;
            else memo[i] = (Integer.parseInt(s.substring(i,i+2))<=26) ? memo[i+1]+memo[i+2] : memo[i+1];
        
        return memo[0];
    }
}

Don't think about other things. Just keep working.
Working hard is the most beautiful moment.
Just keep concentrating on it. --- 10/19/2016