2020-05-10 91. Decode Ways Medium

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

Example 2:

Input:"226"Output:3Explanation:It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

很多时候，动态规划的问题其实不用创建完整的数组，所以可以第一遍先new一个数组，作对了之后，再把new去掉，直接用变量替换需要用到的那几个子问题解即可

class Solution {

    public int numDecodings(String s) {

        int n = s.length();

        int pre1 = 0, pre2 = 0;

        int res = 0;

        int preNum = 0;

        for (int i = 0; i < n; i++) {

            int cur = 0;

            int curNum = s.charAt(i) - '0';

            if (curNum >= 1 && curNum <= 9) cur += i >= 1 ? pre1 : 1;

            int twoNum = 10 * preNum + curNum;

            if (i >= 1 && twoNum >= 10 && twoNum <= 26) cur += i >= 2 ? pre2 : 1;

            if (cur == 0) return 0;

            pre2 = pre1;

            pre1 = cur;

            preNum = curNum;

        }

        return pre1;

    }

}