# Find length of longest substring with at most K normal characters

• Difficulty Level : Medium
• Last Updated : 03 Jun, 2021

Given a string P consisting of small English letters and a 26-digit bit string Q, where 1 represents the special character and 0 represents a normal character for the 26 English alphabets. The task is to find the length of the longest substring with at most K normal characters.

Examples:

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Input : P = “normal”, Q = “00000000000000000000000000”, K=1
Output :
Explanation : In string Q all characters are normal.
Hence, we can select any substring of length 1.

Input : P = “giraffe”, Q = “01111001111111111011111111”, K=2
Output :
Explanation : Normal characters in P from Q are {a, f, g, r}.
Therefore, possible substrings with at most 2 normal characters are {gir, ira, ffe}.
The maximum length of all substring is 3.

Approach:
To solve the problem mentioned above we will be using the concept of two pointers. Hence, maintain left and right pointers of the substring, and a count of normal characters. Increment the right index till the count of normal characters is at most K. Then update the answer with a maximum length of substring encountered till now. Increment left index and decrement count till it is greater than K.
Below is the implementation of the above approach:

## C++

 `// C++ implementation to Find``// length of longest substring``// with at most K normal characters``#include ``using` `namespace` `std;` `// Function to find maximum``// length of normal substrings``int` `maxNormalSubstring(string& P, string& Q,``                       ``int` `K, ``int` `N)``{` `    ``if` `(K == 0)``        ``return` `0;` `    ``// keeps count of normal characters``    ``int` `count = 0;` `    ``// indexes of substring``    ``int` `left = 0, right = 0;` `    ``// maintain length of longest substring``    ``// with at most K normal characters``    ``int` `ans = 0;` `    ``while` `(right < N) {` `        ``while` `(right < N && count <= K) {` `            ``// get position of character``            ``int` `pos = P[right] - ``'a'``;` `            ``// check if current character is normal``            ``if` `(Q[pos] == ``'0'``) {` `                ``// check if normal characters``                ``// count exceeds K``                ``if` `(count + 1 > K)` `                    ``break``;` `                ``else``                    ``count++;``            ``}` `            ``right++;` `            ``// update answer with substring length``            ``if` `(count <= K)``                ``ans = max(ans, right - left);``        ``}` `        ``while` `(left < right) {` `            ``// get position of character``            ``int` `pos = P[left] - ``'a'``;` `            ``left++;` `            ``// check if character is``            ``// normal then decrement count``            ``if` `(Q[pos] == ``'0'``)` `                ``count--;` `            ``if` `(count < K)``                ``break``;``        ``}``    ``}` `    ``return` `ans;``}` `// Driver code``int` `main()``{``    ``// initialise the string``    ``string P = ``"giraffe"``, Q = ``"01111001111111111011111111"``;` `    ``int` `K = 2;` `    ``int` `N = P.length();` `    ``cout << maxNormalSubstring(P, Q, K, N);` `    ``return` `0;``}`

## Java

 `// Java implementation to Find``// length of longest subString``// with at most K normal characters``class` `GFG{`` ` `// Function to find maximum``// length of normal subStrings``static` `int` `maxNormalSubString(``char` `[]P, ``char` `[]Q,``                       ``int` `K, ``int` `N)``{`` ` `    ``if` `(K == ``0``)``        ``return` `0``;`` ` `    ``// keeps count of normal characters``    ``int` `count = ``0``;`` ` `    ``// indexes of subString``    ``int` `left = ``0``, right = ``0``;`` ` `    ``// maintain length of longest subString``    ``// with at most K normal characters``    ``int` `ans = ``0``;`` ` `    ``while` `(right < N) {`` ` `        ``while` `(right < N && count <= K) {`` ` `            ``// get position of character``            ``int` `pos = P[right] - ``'a'``;`` ` `            ``// check if current character is normal``            ``if` `(Q[pos] == ``'0'``) {`` ` `                ``// check if normal characters``                ``// count exceeds K``                ``if` `(count + ``1` `> K)`` ` `                    ``break``;`` ` `                ``else``                    ``count++;``            ``}`` ` `            ``right++;`` ` `            ``// update answer with subString length``            ``if` `(count <= K)``                ``ans = Math.max(ans, right - left);``        ``}`` ` `        ``while` `(left < right) {`` ` `            ``// get position of character``            ``int` `pos = P[left] - ``'a'``;`` ` `            ``left++;`` ` `            ``// check if character is``            ``// normal then decrement count``            ``if` `(Q[pos] == ``'0'``)`` ` `                ``count--;`` ` `            ``if` `(count < K)``                ``break``;``        ``}``    ``}`` ` `    ``return` `ans;``}`` ` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``// initialise the String``    ``String P = ``"giraffe"``, Q = ``"01111001111111111011111111"``;`` ` `    ``int` `K = ``2``;`` ` `    ``int` `N = P.length();`` ` `    ``System.out.print(maxNormalSubString(P.toCharArray(), Q.toCharArray(), K, N));``}``}` `// This code is contributed by Princi Singh`

## Python3

 `# Function to find maximum``# length of normal substrings``def` `maxNormalSubstring(P, Q, K, N):``    ` `    ``if` `(K ``=``=` `0``):``        ``return` `0``  ` `    ``# keeps count of normal characters``    ``count ``=` `0``  ` `    ``# indexes of substring``    ``left, right ``=` `0``, ``0``    ` `    ``# maintain length of longest substring``    ``# with at most K normal characters``    ``ans ``=` `0``  ` `    ``while` `(right < N):``  ` `        ``while` `(right < N ``and` `count <``=` `K):``  ` `            ``# get position of character``            ``pos ``=` `ord``(P[right]) ``-` `ord``(``'a'``)``  ` `            ``# check if current character is normal``            ``if` `(Q[pos] ``=``=` `'0'``):``  ` `                ``# check if normal characters``                ``# count exceeds K``                ``if` `(count ``+` `1` `> K):``                    ``break``                ``else``:``                    ``count ``+``=` `1``  ` `            ``right ``+``=` `1``  ` `            ``# update answer with substring length``            ``if` `(count <``=` `K):``                ``ans ``=` `max``(ans, right ``-` `left)``  ` `        ``while` `(left < right):``  ` `            ``# get position of character``            ``pos ``=` `ord``(P[left]) ``-` `ord``(``'a'``)``  ` `            ``left ``+``=` `1``  ` `            ``# check if character is``            ``# normal then decrement count``            ``if` `(Q[pos] ``=``=` `'0'``):``                ``count ``-``=` `1``  ` `            ``if` `(count < K):``                ``break``  ` `    ``return` `ans``  ` `# Driver code``if``(__name__ ``=``=` `"__main__"``):``    ``# initialise the string``    ``P ``=` `"giraffe"``    ``Q ``=` `"01111001111111111011111111"``  ` `    ``K ``=` `2``  ` `    ``N ``=` `len``(P)``  ` `    ``print``(maxNormalSubstring(P, Q, K, N))` `# This code is contributed by skylags`

## C#

 `// C# implementation to Find``// length of longest subString``// with at most K normal characters``using` `System;` `public` `class` `GFG{` `// Function to find maximum``// length of normal subStrings``static` `int` `maxNormalSubString(``char` `[]P, ``char` `[]Q,``                    ``int` `K, ``int` `N)``{` `    ``if` `(K == 0)``        ``return` `0;` `    ``// keeps count of normal characters``    ``int` `count = 0;` `    ``// indexes of subString``    ``int` `left = 0, right = 0;` `    ``// maintain length of longest subString``    ``// with at most K normal characters``    ``int` `ans = 0;` `    ``while` `(right < N) {` `        ``while` `(right < N && count <= K) {` `            ``// get position of character``            ``int` `pos = P[right] - ``'a'``;` `            ``// check if current character is normal``            ``if` `(Q[pos] == ``'0'``) {` `                ``// check if normal characters``                ``// count exceeds K``                ``if` `(count + 1 > K)` `                    ``break``;` `                ``else``                    ``count++;``            ``}` `            ``right++;` `            ``// update answer with subString length``            ``if` `(count <= K)``                ``ans = Math.Max(ans, right - left);``        ``}` `        ``while` `(left < right) {` `            ``// get position of character``            ``int` `pos = P[left] - ``'a'``;` `            ``left++;` `            ``// check if character is``            ``// normal then decrement count``            ``if` `(Q[pos] == ``'0'``)` `                ``count--;` `            ``if` `(count < K)``                ``break``;``        ``}``    ``}` `    ``return` `ans;``}` `// Driver code``public` `static` `void` `Main(String[] args)``{``    ``// initialise the String``    ``String P = ``"giraffe"``, Q = ``"01111001111111111011111111"``;` `    ``int` `K = 2;` `    ``int` `N = P.Length;` `    ``Console.Write(maxNormalSubString(P.ToCharArray(),``                     ``Q.ToCharArray(), K, N));``}``}` `// This code contributed by Princi Singh`

## Javascript

 ``
Output:
`3`

Time Complexity: The above method takes O(N) time.

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