Count And Say

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the n^th sequence.

Note: The sequence of integers will be represented as a string.

This question is a typical DP problem. The nth sequence can be calculated from the (n-1)th sequence. Therefore, we just store the result of each step.

Java: CountAndSay

public String countAndSay(int n) {
   String []sequence=new String[n];
   sequence[0]="1";
 
   for(int i=1;i<n;i++)
   {
       char[] str=sequence[i-1].toCharArray();
       StringBuffer sb=new StringBuffer();
       int start_value=str[0]-'0';
       int j=1;
       int counter=1;
       while(j<str.length)
       {
           if(str[j]-'0'==start_value)
           {
               counter++;
           }
           else
           {
               sb.append(counter);
               sb.append(start_value);
               start_value=str[j]-'0';
               counter=1;
           }
           j++;
       }
       sb.append(counter);
       sb.append(start_value);
       sequence[i]=sb.toString();
   }
 
   return sequence[n-1];
}