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DecodeWays.java
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DecodeWays.java
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package Leetcode;
/**
* @author kalpak
*
* A message containing letters from A-Z can be encoded into numbers using the following mapping:
*
* 'A' -> "1"
* 'B' -> "2"
* ...
* 'Z' -> "26"
* To decode an encoded message,
* all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways).
*
* For example, "11106" can be mapped into:
* "AAJF" with the grouping (1 1 10 6)
* "KJF" with the grouping (11 10 6)
* Note that the grouping (1 11 06) is invalid because "06" cannot be mapped into 'F' since "6" is different from "06".
*
* Given a string s containing only digits, return the number of ways to decode it.
*
* The answer is guaranteed to fit in a 32-bit integer.
*
* Example 1:
* Input: s = "12"
* Output: 2
* Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).
*
* Example 2:
* Input: s = "226"
* Output: 3
* Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
*
* Example 3:
*
* Input: s = "0"
* Output: 0
* Explanation: There is no character that is mapped to a number starting with 0.
* The only valid mappings with 0 are 'J' -> "10" and 'T' -> "20", neither of which start with 0.
* Hence, there are no valid ways to decode this since all digits need to be mapped.
* Example 4:
*
* Input: s = "06"
* Output: 0
* Explanation: "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").
*
*
* Constraints:
*
* 1 <= s.length <= 100
* s contains only digits and may contain leading zero(s).
*/
public class DecodeWays {
public static int numDecodings(String s) {
Integer[] dp = new Integer[s.length()];
return decodedStringRecursive(s, dp, 0);
}
private static int decodedStringRecursive(String s, Integer[] dp, int currentIdx) {
if(currentIdx >= s.length())
return 1;
if(s.charAt(currentIdx) == '0') // if 0 is in the middle number
return 0;
if(dp[currentIdx] != null)
return dp[currentIdx];
// A character may be decoded alone or by pairing with the next char.
int ways1 = decodedStringRecursive(s, dp, currentIdx + 1);
int ways2= 0;
if(currentIdx + 2 <= s.length() && Integer.parseInt(s.substring(currentIdx, currentIdx + 2)) <= 26)
ways2 = decodedStringRecursive(s, dp, currentIdx + 2);
dp[currentIdx] = ways1 + ways2;
return dp[currentIdx];
}
public static int decodeStringBottomsUp(String s) {
if(s == null || s.length() == 0)
return 0;
int[] dp = new int[s.length() + 1];
dp[0] = 1; // empty string
dp[1] = s.charAt(0) == '0' ? 0 : 1; // 1st character in string
for(int i = 2; i <= s.length(); i++) {
if(s.charAt(i - 1) != '0')
dp[i] += dp[i - 1]; // just append the character
int twoDigit = Integer.parseInt(s.substring(i - 2, i));
if(twoDigit >= 10 && twoDigit <= 26)
dp[i] += dp[i - 2];
}
return dp[dp.length - 1];
}
public static void main(String[] args) {
System.out.println(numDecodings("12"));
System.out.println(decodeStringBottomsUp("226"));
}
}