[Leetcode C++] Count and Say

Problem

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     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 term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

Analysis

This problem is about string operation

Solution

class Solution {
public:
    string countAndSay(int n) {
        string num = "1";
        
        //interatively read n times
        for(int i=1; i<n; i++){
            num = helper(num);
        }
        
        return num;
    }
    
    //helper function for reading a number
    string helper(string num){
        
        stringstream nextStr;
        char last = num[0];
        int count = 1;
        for(int i=1; i<num.size(); i++){
            if(num[i] == last){
                count++;
            }else{
                nextStr<<count<<last;
                last = num[i];
                count = 1;
            }
        }
        nextStr<<count<<last;
        return nextStr.str();
    }
};

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