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: 2 Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Example 2:
Input: "226" Output: 3 Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
用动态规划解决:
public int numDecodings(String s) {
int[] dp = new int[s.length()+1];//dp[i]表示s[0,i-1]有多少种划分方法
dp[0] = 1;//为了dp[2]
dp[1] = s.charAt(0)!='0'?1:0;
for(int i = 2;i<dp.length;i++){//dp[i]表示s[0,i-1]
char one = s.charAt(i-1);
if(one!='0'){
dp[i] += dp[i-1];
}
int two = Integer.valueOf(s.substring(i-2,i));//s[i-2,i-1]
if(two>=10 && two<=26){
dp[i] += dp[i-2];
}
}
return dp[dp.length-1];
}