Leetcode: 91. Decode Ways

Description

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.

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

• 动态规划
• 对每个数组首先判断其是否为0，若是将改为dp赋0，若不是，赋上一个dp值，此时相当如加上了dp[i - 1], 然后看数组前一位是否存在，如果存在且满足前一位不是0，且和当前为一起组成的两位数不大于26，则当前dp值加上dp[i - 2],

代码

class Solution {
public:
    int numDecodings(string s) {
        int len = s.size();
        if(len == 0) return 0;
        
        vector<int> dp(len + 1, 0);
        dp[0] = 1;
        
        
        for(int i = 1; i < len + 1; ++i){
            dp[i] = (s[i - 1] == '0') ? 0 : dp[i - 1];
            
            if(i > 1 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6')))
                dp[i] += dp[i - 2];
        }
        
        return dp[len];
    }
};