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DecodeWays.java
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DecodeWays.java
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/**
* Practiced on 12/3/2012
*
* A message containing letters from A-Z is being encoded to numbers using the following mapping:
*
* 'A' -> 1
* 'B' -> 2
* ...
* 'Z' -> 26
*
* Given an encoded message containing digits, determine the total number of ways to decode it.
*
* For example,
* Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
*
* The number of ways decoding "12" is 2.
*
* ==============================
* Dynamic programming is used. It is very tricky to determine the first two elements.
* You need to be very careful with '0'.
*/
package com.congli.leetcode;
public class DecodeWays {
public int numDecodings(String s)
{
if(s == null || s.length() == 0) return 0;
if(s.charAt(0) == '0') return 0;
int[] resultArray = new int[s.length()];
resultArray[0] = 1;
if(s.length() == 1) return resultArray[0];
if(validTwoDigits(s.substring(0, 2)))
{
resultArray[1] = s.charAt(1) == '0' ? 1 : 2;
}
else
{
resultArray[1] = s.charAt(1) == '0' ? 0 : 1;
}
for(int i = 2; i < s.length(); ++i)
{
if(s.charAt(i) != '0')
{
resultArray[i] += resultArray[i-1];
}
if(validTwoDigits(s.substring(i-1, i+1)))
{
resultArray[i] += resultArray[i-2];
}
if(resultArray[i] == 0)
return 0;
}
return resultArray[resultArray.length-1];
}
private boolean validTwoDigits(String str)
{
int num = Integer.parseInt(str);
if(num <= 26 && num >= 10)
return true;
else
return false;
}
}