# Look-and-Say Sequence

• Difficulty Level : Medium
• Last Updated : 21 Jun, 2021

Find the n’th term in Look-and-say (Or Count and Say) Sequence. The look-and-say sequence is the sequence of below integers:
1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, …

How is above sequence generated?
n’th term in generated by reading (n-1)’th term.

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```The first term is "1"

Second term is "11", generated by reading first term as "One 1"
(There is one 1 in previous term)

Third term is "21", generated by reading second term as "Two 1"

Fourth term is "1211", generated by reading third term as "One 2 One 1"

and so on```

How to find n’th term?

Example:

```Input: n = 3
Output: 21

Input: n = 5
Output: 111221```

The idea is simple, we generate all terms from 1 to n. First two terms are initialized as “1” and “11”, and all other terms are generated using previous terms. To generate a term using previous term, we scan the previous term. While scanning a term, we simply keep track of count of all consecutive characters. For sequence of same characters, we append the count followed by character to generate the next term.

Below is implementation of above idea.

## C++

 `// C++ program to find n'th term in look and say``// sequence``#include ``using` `namespace` `std;` `// Returns n'th term in look-and-say sequence``string countnndSay(``int` `n)``{``    ``// Base cases``    ``if` `(n == 1)      ``return` `"1"``;``    ``if` `(n == 2)      ``return` `"11"``;` `    ``// Find n'th term by generating all terms from 3 to``    ``// n-1.  Every term is generated using previous term``    ``string str = ``"11"``; ``// Initialize previous term``    ``for` `(``int` `i = 3; i<=n; i++)``    ``{``        ``// In below for loop, previous character``        ``// is processed in current iteration. That``        ``// is why a dummy character is added to make``        ``// sure that loop runs one extra iteration.``        ``str += ``'\$'``;``        ``int` `len = str.length();` `        ``int` `cnt = 1; ``// Initialize count of matching chars``        ``string  tmp = ``""``; ``// Initialize i'th term in series` `        ``// Process previous term to find the next term``        ``for` `(``int` `j = 1; j < len; j++)``        ``{``            ``// If current character does't match``            ``if` `(str[j] != str[j-1])``            ``{``                ``// Append count of str[j-1] to temp``                ``tmp += cnt + ``'0'``;` `                ``// Append str[j-1]``                ``tmp += str[j-1];` `                ``// Reset count``                ``cnt = 1;``            ``}` `            ``//  If matches, then increment count of matching``            ``// characters``            ``else` `cnt++;``        ``}` `        ``// Update str``        ``str = tmp;``    ``}` `    ``return` `str;``}` `// Driver program``int` `main()``{``    ``int` `N = 3;``    ``cout << countnndSay(N) << endl;``    ``return` `0;``}`

## Java

 `// Java program to find n'th``// term in look and say sequence` `class` `GFG``{` `    ``// Returns n'th term in``    ``// look-and-say sequence``    ``static` `String countnndSay(``int` `n)``    ``{``    ``// Base cases``    ``if` `(n == ``1``)     ``return` `"1"``;``    ``if` `(n == ``2``)     ``return` `"11"``;` `    ``// Find n'th term by generating``    ``// all terms from 3 to n-1.``    ``// Every term is generated``    ``// using previous term``    ` `    ``// Initialize previous term``    ``String str = ``"11"``;``    ``for` `(``int` `i = ``3``; i <= n; i++)``    ``{``        ``// In below for loop, previous``        ``// character is processed in``        ``// current iteration. That is``        ``// why a dummy character is``        ``// added to make sure that loop``        ``// runs one extra iteration.``        ``str += ``'\$'``;``        ``int` `len = str.length();` `        ``int` `cnt = ``1``; ``// Initialize count``                     ``// of matching chars``        ``String tmp = ``""``; ``// Initialize i'th``                         ``// term in series``        ``char` `[]arr = str.toCharArray();``        ` `        ``// Process previous term``        ``// to find the next term``        ``for` `(``int` `j = ``1``; j < len; j++)``        ``{``            ``// If current character``            ``// does't match``            ``if` `(arr[j] != arr[j - ``1``])``            ``{``                ``// Append count of``                ``// str[j-1] to temp``                ``tmp += cnt + ``0``;` `                ``// Append str[j-1]``                ``tmp += arr[j - ``1``];` `                ``// Reset count``                ``cnt = ``1``;``            ``}` `            ``// If matches, then increment``            ``// count of matching characters``            ``else` `cnt++;``        ``}` `        ``// Update str``        ``str = tmp;``    ``}` `    ``return` `str;``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int` `N = ``3``;``        ``System.out.println(countnndSay(N));``    ``}``}` `// This code is contributed``// by ChitraNayal`

## C#

 `// C# program to find n'th``// term in look and say sequence``using` `System;` `class` `GFG``{` `    ``// Returns n'th term in``    ``// look-and-say sequence``    ``static` `string` `countnndSay(``int` `n)``    ``{``    ``// Base cases``    ``if` `(n == 1)     ``return` `"1"``;``    ``if` `(n == 2)     ``return` `"11"``;` `    ``// Find n'th term by generating``    ``// all terms from 3 to n-1.``    ``// Every term is generated using``    ``// previous term``    ` `    ``// Initialize previous term``    ``string` `str = ``"11"``;``    ``for` `(``int` `i = 3; i <= n; i++)``    ``{``        ``// In below for loop, previous``        ``// character is processed in``        ``// current iteration. That is``        ``// why a dummy character is``        ``// added to make sure that loop``        ``// runs one extra iteration.``        ``str += ``'\$'``;``        ``int` `len = str.Length;` `        ``int` `cnt = 1; ``// Initialize count of``                     ``// matching chars``        ``string` `tmp = ``""``; ``// Initialize i'th``                         ``// term in series``        ``char` `[]arr = str.ToCharArray();``        ` `        ``// Process previous term``        ``// to find the next term``        ``for` `(``int` `j = 1; j < len; j++)``        ``{``            ``// If current character``            ``// does't match``            ``if` `(arr[j] != arr[j - 1])``            ``{``                 ``// Append count of``                ``// str[j-1] to temp``                ``tmp += cnt + 0;` `                ``// Append str[j-1]``                ``tmp += arr[j - 1];` `                ``// Reset count``                ``cnt = 1;``            ``}` `            ``// If matches, then increment``            ``// count of matching characters``            ``else` `cnt++;``        ``}` `        ``// Update str``        ``str = tmp;``    ``}` `    ``return` `str;``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `Main()``    ``{``        ``int` `N = 3;``        ``Console.Write(countnndSay(N));``    ``}``}` `// This code is contributed``// by ChitraNayal`

## Python3

 `# Python 3 program to find``# n'th term in look and``# say sequence` `# Returns n'th term in``# look-and-say sequence``def` `countnndSay(n):``    ` `    ``# Base cases``    ``if` `(n ``=``=` `1``):``        ``return` `"1"``    ``if` `(n ``=``=` `2``):``        ``return` `"11"` `    ``# Find n'th term by generating``    ``# all terms from 3 to n-1.``    ``# Every term is generated using``    ``# previous term``    ` `    ``# Initialize previous term``    ``s ``=` `"11"``    ``for` `i ``in` `range``(``3``, n ``+` `1``):``        ` `        ``# In below for loop,``        ``# previous character is``        ``# processed in current``        ``# iteration. That is why``        ``# a dummy character is``        ``# added to make sure that``        ``# loop runs one extra iteration.``        ``s ``+``=` `'\$'``        ``l ``=` `len``(s)` `        ``cnt ``=` `1` `# Initialize count``                ``# of matching chars``        ``tmp ``=` `"" ``# Initialize i'th``                 ``# term in series` `        ``# Process previous term to``        ``# find the next term``        ``for` `j ``in` `range``(``1` `, l):``            ` `            ``# If current character``            ``# does't match``            ``if` `(s[j] !``=` `s[j ``-` `1``]):``                ` `                ``# Append count of``                ``# str[j-1] to temp``                ``tmp ``+``=` `str``(cnt ``+` `0``)` `                ``# Append str[j-1]``                ``tmp ``+``=` `s[j ``-` `1``]` `                ``# Reset count``                ``cnt ``=` `1``            ` `            ``# If matches, then increment``            ``# count of matching characters``            ``else``:``                ``cnt ``+``=` `1` `        ``# Update str``        ``s ``=` `tmp``    ``return` `s;` `# Driver Code``N ``=` `3``print``(countnndSay(N))` `# This code is contributed``# by ChitraNayal`

## PHP

 ``

## Javascript

 ``
Output
```21
```

Another Approach(Using STL): There is one more idea where we can use unordered_map from c++ stl to track count of digits. Basic idea is to use a generator function which will generate string from the previous string . In the count and say function we will iterate over integers from 1 to n-1 and keep updating our result .

## C++

 `#include ``using` `namespace` `std;` `// generator function returns int string from prev int``// string e.g. -> it will return '1211' for '21' ( One 2's``// and One 1)``string generator(string str)``{``    ``string ans = ``""``;` `    ``unordered_map<``char``, ``int``>``        ``tempCount; ``// It is used to count integer sequence` `    ``for` `(``int` `i = 0; i < str.length() + 1; i++) {``        ``// when current char is different from prev one we``        ``// clear the map and update the ans``        ``if` `(tempCount.find(str[i]) == tempCount.end()``            ``&& i > 0) {``            ``auto` `prev = tempCount.find(str[i - 1]);``            ``ans += to_string(prev->second) + prev->first;``            ``tempCount.clear();``        ``}``        ``// when current char is same as prev one we increase``        ``// it's count value``        ``tempCount[str[i]]++;``    ``}` `    ``return` `ans;``}` `string countnndSay(``int` `n)``{``    ``string res = ``"1"``; ``// res variable keep tracks of string``                      ``// from 1 to n-1` `    ``// For loop iterates for n-1 time and generate strings``    ``// in sequence "1" -> "11" -> "21" -> "1211"``    ``for` `(``int` `i = 1; i < n; i++) {``        ``res = generator(res);``    ``}` `    ``return` `res;``}` `int` `main()``{` `    ``int` `N = 3;``    ``cout << countnndSay(N) << endl;``    ``return` `0;``}`
Output
```21
```

Thanks to Utkarsh and Neeraj for suggesting the above solution.