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:
Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
解释下题目:
给定一串数字组成的字符串,看看有多少种字母组合与之对应
1. 动态规划
实际耗时:5ms
public int numDecodings(String s) {
if (s == null || s.length() == 0) {
return 0;
}
int n = s.length();
int[] dp = new int[n + 1];
dp[0] = 1;
dp[1] = s.charAt(0) == '0' ? 0 : 1;
for (int i = 2; i <= n; i++) {
int num1 = Integer.valueOf(s.substring(i - 1, i));
int num2 = Integer.valueOf(s.substring(i - 2, i));
if (num1 >= 1 && num1 <= 9) {
dp[i] += dp[i - 1];
}
if (num2 >= 10 && num2 <= 26) {
dp[i] += dp[i - 2];
}
}
return dp[n];
}
思路:对于每一个位子,仅需要考虑两种情况:1.如果该位子的值是1-9,那么说明它可以单独作为一个编码;2.如果它和之前的一位组合在一起,可以是10-26之间(包含10和26),那么说明它还可以和之前的前一格进行组合。