Count and Say

Source

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 nth sequence.

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Example
Given n = 5, return "111221".

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

Python

class Solution:
    # @param {int} n the nth
    # @return {string} the nth sequence
    def countAndSay(self, n):
        # Write your code here

        result="1"
        for _ in range(1, n):
            tmp=[]
            count=1
            for i in range(1, len(result)):
                if result[i]==result[i-1]:
                    count+=1
                else:
                    tmp.append(str(count))
                    tmp.append(result[i-1])
                    count=1
            tmp.append(str(count))
            tmp.append(result[-1])
            result = ''.join(tmp)

        return result

Java

public class Solution {
    /**
     * @param n the nth
     * @return the nth sequence
     */
    public String countAndSay(int n) {
        if(n<=0){
            return null;
        }
        StringBuilder result = new StringBuilder();
        result.append("1");
        for(int j=1; j<n; j++){
            StringBuilder tmp = new StringBuilder();
            int count=1;
            for(int i=1; i<result.length(); i++){
                if(result.charAt(i)==result.charAt(i-1)){
                    count++;
                }
                else{
                    tmp.append(count);
                    tmp.append(result.charAt(i-1));
                    count=1;
                }
            }
            tmp.append(count);
            tmp.append(result.charAt(result.length()-1));
            result=tmp;
        }
        return result.toString();
    }
}