Difficulty: Medium
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).
Language: Java
class Solution {
public int numDecodings(String s) { // 动态规划
if (s == null || s.length() == 0) {
return 0;
}
int[] dp = new int[s.length() + 1];
dp[0] = 1;
dp[1] = s.charAt(0) != '0' ? 1 : 0;
for (int i = 2; i <= s.length(); i++) {
int first = Integer.parseInt(s.substring(i - 1, i));
int second = Integer.parseInt(s.substring(i - 2, i));
if (first > 0 && first < 10) {
dp[i] += dp[i - 1];
}
if (second >= 10 && second <= 26) {
dp[i] += dp[i - 2];
}
}
return dp[s.length()];
}
}Language: Java
class Solution {
public int numDecodings(String s) { // 动态规划
if (s.startsWith("0")) {
return 0;
}
return this.numDecodings(s.toCharArray(), s.length(), new int[s.length()]);
}
private int numDecodings(char[] chars, int length, int[] dp) {
if (length <= 1) {
return 1;
}
if (dp[length - 1] > 0) {
return dp[length - 1];
}
int num = 0;
if (chars[length - 1] != '0') {
num += numDecodings(chars, length - 1, dp);
} else {
if (chars[length - 2] == '0') {
dp[length - 1] = num;
return num;
}
}
if (chars[length - 2] != '0' && (chars[length - 2] - '0') * 10 + chars[length - 1] - '0' <= 26) {
num += numDecodings(chars, length - 2, dp);
}
dp[length - 1] = num;
return num;
}
}