leetcode: Decode Ways

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.

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class Solution {
public:
int numDecodings(string s) {

if (s.size() == 0)
return 0;
vector<int> dp(s.size()+1);

dp[0] = 1;
if (s[0] != '0')
dp[1] = 1;
else
dp[1] = 0; //一定要注意这个条件

for (int i=1; i<s.size(); i++)
{
if (s[i] != '0')
dp[i+1] = dp[i];

if ((s[i]>='0' && s[i] <='6' && s[i-1] =='2') || (s[i]>='0' && s[i]<='9' && s[i-1]=='1'))
{
dp[i+1] += dp[i-1];
}
}

return dp[s.size()];
}
};