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88 lines (70 loc) · 2.36 KB
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/*
91. Decode Ways (https://leetcode.com/problems/decode-ways)
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).
*/
package recusion;
import java.util.ArrayList;
import java.util.List;
//这道题的解法一和 [131. Palindrome Partitioning] 相同
public class DecodeWays {
//解法一 DFS同 131.Palindrome Partitioning
public static int decodeWaysS1(String s) {
List<List<String>> list = new ArrayList<>();
createItem(list, new ArrayList<String>(), s, 0);
System.out.println(list);
return list.size();
}
private static void createItem(List<List<String>> list, List<String> tempList, String s, int start) {
if (start > s.length() - 1) {
list.add(new ArrayList<>(tempList));
return;
}
for (int i = start; i < s.length(); i++) {
String sub = s.substring(start, i + 1);
if (Integer.parseInt(sub) <= 26) {
tempList.add(sub);
createItem(list, tempList, s, i + 1);
tempList.remove(tempList.size() - 1);
}
}
}
//解法二 动态规划 DP
//DP解法相似题：
// 62. Unique Paths (https://leetcode.com/problems/unique-paths/)
//70. Climbing Stairs (https://leetcode.com/problems/climbing-stairs/)
//509. Fibonacci Number (https://leetcode.com/problems/fibonacci-number/)
public static 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 first = Integer.parseInt(s.substring(i - 1, i));
int second = Integer.parseInt(s.substring(i - 2, i));
if (first > 0 && first < 9) {
dp[i] += dp[i - 1];
}
if (second > 10 && second < 26) {
dp[i] += dp[i - 2];
}
}
return dp[n];
}
}