LeetCode 91. Decode Ways（解码方法）

原题网址：https://leetcode.com/problems/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.

方法：动态规划。

public class Solution {
public int numDecodings(String s) {
if (s == null || s.length() == 0) return 0;
char[] sa = s.toCharArray();
int[] nums = new int[sa.length+1];
nums[0] = 1;
for(int i=1; i<=sa.length; i++) {
if (sa[i-1] != '0') nums[i] += nums[i-1];
if (i>1 && sa[i-2] == '1') nums[i] += nums[i-2];
if (i>1 && sa[i-2] == '2' && sa[i-1] >= '0' && sa[i-1] <= '6') nums[i] += nums[i-2];
}
return nums[sa.length];
}
}