DecodeWays

DecodeWays/DecodeWays.h

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+/*
+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.
+*/
+
+#include <map>
+#include <string>
+using namespace std;
+
+class Solution {
+public:
+	int numDecodings(string s)
+	{
+		int n=s.length();
+		if(n==0)
+			return 0;
+
+		count.clear();
+		count[n]=1;
+		if(s[n-1]=='0')
+			count[n-1]=0;
+		else
+			count[n-1]=1;
+		for(int i=n-2;i>=0;--i)
+		{
+			char cur=s[i]-'0';
+			char next=s[i+1]-'0';
+			if(cur==0)
+			{
+				count[i]=0;
+			}
+			else if(cur==1)
+			{
+				count[i]=count[i+1]+count[i+2];
+			}
+			else if(cur==2)
+			{
+				if(next<=6)
+					count[i]=count[i+1]+count[i+2];
+				else
+					count[i]=count[i+1];
+			}
+			else
+			{
+				count[i]=count[i+1];
+			}
+		}
+		return count[0];
+	}
+	void test()
+	{
+		string s;
+		cin>>s;
+		cout<<numDecodings(s);
+	}
+private:
+	map<int,int> count;
+};