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.
1 public class Solution { 2 public int numDecodings(String s) { 3 int num = 0; 4 if(s.length() > 0){ 5 int[] decodeways = new int[s.length()]; 6 if(Integer.parseInt(s.substring(0, 1)) == 0) 7 return 0; 8 else 9 decodeways[0] = 1; 10 if(s.length()> 1){ 11 if(Integer.parseInt(s.substring(1, 2)) != 0){ 12 if(Integer.parseInt(s.substring(0, 2)) > 26) 13 decodeways[1] = 1; 14 else{ 15 if(Integer.parseInt(s.substring(0, 1)) == 0) 16 return 0; 17 else 18 decodeways[1] = 2; 19 } 20 } 21 else{ 22 if(Integer.parseInt(s.substring(0, 1)) > 2) 23 return 0; 24 else 25 decodeways[1] = 1; 26 } 27 for(int i = 2; i < s.length(); ++i){ 28 if(Integer.parseInt(s.substring(i, i + 1)) != 0){ 29 if(Integer.parseInt(s.substring(i - 1, i + 1)) > 26 || 30 Integer.parseInt(s.substring(i - 1, i)) == 0) 31 decodeways[i] = decodeways[i - 1]; 32 else 33 decodeways[i] = decodeways[i - 1] + decodeways[i - 2]; 34 } 35 else{ 36 if(Integer.parseInt(s.substring(i - 1, i)) == 0 || 37 Integer.parseInt(s.substring(i - 1, i)) > 2){ 38 num = 0; 39 break; 40 } 41 else 42 decodeways[i] = decodeways[i - 2]; 43 } 44 } 45 } 46 num = decodeways[s.length() - 1]; 47 } 48 return num; 49 } 50 }
code 2, the same algorithm as above
1 public class Solution { 2 public int numDecodings(String s) { 3 int num = 0; 4 if(s.length() == 0 || s.charAt(0) == '0') 5 return num; 6 else{ 7 int[] ways = new int[s.length() + 1]; 8 ways[0] = ways[1] = 1; 9 for(int i = 2; i < s.length() + 1; ++i){ 10 int cur = Integer.parseInt(s.substring(i - 1, i)); 11 int pre = Integer.parseInt(s.substring(i - 2, i - 1)); 12 if(cur != 0){ 13 if(pre != 0){ 14 ways[i] = (Integer.parseInt(s.substring(i - 2, i)) < 27) ? ways[i - 1] + ways[i - 2] : ways[i - 1]; 15 } 16 else 17 ways[i] = ways[i - 2]; 18 } 19 else{ 20 if(pre > 2 || pre == 0) 21 return num; 22 else 23 ways[i] = ways[i - 2]; 24 } 25 } 26 num = ways[s.length()]; 27 } 28 return num; 29 } 30 }
The third code:
public class Solution {
public int numDecodings(String s) {
int result = 0;
int length = s.length();
if(length > 0){
int[] ways = new int[length + 1];
ways[length] = 1;
ways[length - 1] = (s.charAt(length - 1) - '0' == 0) ? 0 : 1;
for(int i = length - 2; i > -1; --i){
if(s.charAt(i)-'0' == 0)
ways[i] = 0;
else if((s.charAt(i) - '0' == 1)){
if(s.charAt(i + 1) - '0' == 0)
ways[i] = ways[i + 2];
else
ways[i] = ways[i + 2] + ways[i + 1];
}
else if(s.charAt(i) -'0' == 2){
if(s.charAt(i + 1) - '0' == 0)
ways[i] = ways[i + 2];
else if(s.charAt(i + 1) - '0' != 0 && s.charAt(i + 1) - '0' < 7)
ways[i] = ways[i + 1] + ways[i + 2];
else
ways[i] = ways[i + 1];
}
else{
if(s.charAt(i + 1) - '0' == 0)
ways[i] = 0;
else
ways[i] = ways[i + 1];
}
}
result = ways[0];
}
return result;
}
}