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65 changes: 65 additions & 0 deletionsjava/91-Decode-Ways.java
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,65 @@ | ||
| // Optimal | ||
| class Solution { | ||
| public int numDecodings(String s){ | ||
| int[] dp = new int[s.length()+1]; | ||
| int twoBack = 1; // empty string | ||
| int oneBack = s.charAt(0) == '0' ? 0: 1; | ||
| int current = oneBack; | ||
| for(int i = 2; i < s.length()+1; i++){ | ||
| current = 0; | ||
| if(s.charAt(i-1) != '0'){ | ||
| current += oneBack; | ||
| } | ||
| if(s.charAt(i-2) == '1' || (s.charAt(i-2) == '2' && s.charAt(i-1) < '7')){ | ||
| current += twoBack; | ||
| } | ||
| twoBack = oneBack; | ||
| oneBack = current; | ||
| } | ||
| return current; | ||
| } | ||
| } | ||
| //bottom up | ||
| class Solution { | ||
| public int numDecodings(String s){ | ||
| int[] dp = new int[s.length()+1]; | ||
| dp[0] = 1; // empty string | ||
| dp[1] = s.charAt(0) == '0' ? 0: 1; | ||
| for(int i = 2; i < s.length()+1; i++){ | ||
| if(s.charAt(i-1) != '0'){ | ||
| dp[i] += dp[i-1]; | ||
| } | ||
| if(s.charAt(i-2) == '1' || (s.charAt(i-2) == '2' && s.charAt(i-1) < '7')){ | ||
| dp[i] += dp[i-2]; | ||
| } | ||
| } | ||
| return dp[s.length()]; | ||
| } | ||
| } | ||
| //top down with memoization | ||
| class Solution { | ||
| public int numDecodings(String s){ | ||
| return numDecodings(s,0,new Integer[s.length()]); | ||
| } | ||
| private int numDecodings(String s, int i,Integer[] dp){ | ||
| if(i == s.length()) return 1; | ||
| if(s.charAt(i) == '0') return 0; | ||
| if(dp[i] != null) return dp[i]; | ||
| int count = 0; | ||
| count += numDecodings(s,i+1,dp); | ||
| if(i < s.length()-1 && (s.charAt(i) == '1' || s.charAt(i) == '2' && s.charAt(i+1) < '7')){ | ||
| count += numDecodings(s,i+2,dp); | ||
| } | ||
| dp[i] = count; | ||
| return dp[i]; | ||
| } | ||
| } | ||
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