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 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.

思路：使用动态规划。子问题为更小的子列可以有多少种分法。每当判断一个新的列时，判断新加入的元素能不能和它相邻的元素组成一个不大于26的数，若可以则分法增加，否则分法不变。注意元素为0的情况分法不变。

class Solution {public:    int numDecodings(string s) {        if (s == "") {            return 0;        }        int result[s.length() + 1];        for (int i = 0; i < s.length() + 1; i++) {            result[i] = 0;        }        result[s.length()] = 1;        for (int i = s.length() - 1; i >= 0; i--) {            if (i == s.length() - 1) {                if (s[i] > '0') {                    result[i] = 1;                }            }            else {                if (s[i] > '0') {                    result[i] = result[i + 1];                    string temp = "";                    temp += s[i];                    temp += s[i + 1];                    if (temp <= "26") {                        result[i] += result[i + 2];                    }                 }            }        }        return result[0];    }};