# Longest substring that starts with X and ends with Y

Given a string str, two characters X and Y. The task is to find the length of the longest substring that starts with X and ends with Y. It is given that there always exists a substring that starts with X and ends with Y
Examples:

Input: str = “QWERTYASDFZXCV”, X = ‘A’, Y = ‘Z’
Output:
Explanation:
The largest substring which start with ‘A’ and end with ‘Z’ = “ASDFZ”.
Size of the substring = 5.

Input: str = “ZABCZ”, X = ‘Z’, Y = ‘Z’
Output:
Explanation:
The largest substring which start with ‘Z’ and end with ‘Z’ = “ZABCZ”.
Size of the substring = 5.

Naive Approach: The naive approach is to find all the substrings of the given string out of these find the largest substring which starts with X and ends with Y
Time Complexity: O(N2
Auxillary Space: O(1)

Efficient Approach: To optimized the above approach, the count of characters between X and Y should be the largest. So, iterate over the string using pointers start and end to find the first occurrence of X from the starting index and the last occurrence of Y from the end. Below are the steps:

1. Initialize start = 0 and end = length of string – 1.
2. Traverse the string from the beginning and find the first occurrence of character X. Let it be at index xPos.
3. Traverse the string from the beginning and find the last occurrence of character Y. Let it be at index yPos.
4. The length of the longest substring is given by (yPos – xPos + 1).

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach  ` `#include   ` `using` `namespace` `std;  ` ` `  `// Function returns length of longest  ` `// substring starting with X and  ` `// ending with Y  ` `int` `longestSubstring(string str,  ` `                    ``char` `X, ``char` `Y)  ` `{  ` `    ``// Length of string  ` `    ``int` `N = str.length();  ` `    ``int` `start = 0;  ` `    ``int` `end = N - 1;  ` `    ``int` `xPos = 0;  ` `    ``int` `yPos = 0;  ` ` `  `    ``// Find the length of the string  ` `    ``// starting with X from the beginning  ` `    ``while` `(``true``) {  ` ` `  `        ``if` `(str[start] == X) {  ` `            ``xPos = start;  ` `            ``break``;  ` `        ``}  ` `        ``start++;  ` `    ``}  ` ` `  `    ``// Find the length of the string  ` `    ``// ending with Y from the end  ` `    ``while` `(``true``) {  ` ` `  `        ``if` `(str[end] == Y) {  ` `            ``yPos = end;  ` `            ``break``;  ` `        ``}  ` `        ``end--;  ` `    ``}  ` ` `  `    ``// Longest substring  ` `    ``int` `length = (yPos - xPos) + 1;  ` ` `  `    ``// Print the length  ` `    ``cout << length;  ` `}  ` ` `  `// Driver Code  ` `int` `main()  ` `{  ` `    ``// Given string str  ` `    ``string str = ``"HASFJGHOGAKZZFEGA"``;  ` ` `  `    ``// Starting and Ending characters  ` `    ``char` `X = ``'A'``, Y = ``'Z'``;  ` ` `  `    ``// Function Call  ` `    ``longestSubstring(str, X, Y);  ` `    ``return` `0;  ` `}  `

## Java

 `// Java program for the above approach  ` `class` `GFG{ ` `     `  `// Function returns length of longest  ` `// substring starting with X and  ` `// ending with Y  ` `public` `static` `void` `longestSubstring(String str,  ` `                                    ``char` `X, ``char` `Y)  ` `{  ` `     `  `    ``// Length of string  ` `    ``int` `N = str.length();  ` `    ``int` `start = ``0``;  ` `    ``int` `end = N - ``1``;  ` `    ``int` `xPos = ``0``;  ` `    ``int` `yPos = ``0``;  ` `     `  `    ``// Find the length of the string  ` `    ``// starting with X from the beginning  ` `    ``while` `(``true``) ` `    ``{  ` `        ``if` `(str.charAt(start) == X) ` `        ``{  ` `            ``xPos = start;  ` `            ``break``;  ` `        ``}  ` `        ``start++;  ` `    ``}  ` `     `  `    ``// Find the length of the string  ` `    ``// ending with Y from the end  ` `    ``while` `(``true``) ` `    ``{  ` `        ``if` `(str.charAt(end) == Y) ` `        ``{  ` `            ``yPos = end;  ` `            ``break``;  ` `        ``}  ` `        ``end--;  ` `    ``}  ` `     `  `    ``// Longest substring  ` `    ``int` `length = (yPos - xPos) + ``1``;  ` `     `  `    ``// Print the length  ` `    ``System.out.print(length); ` `}  ` ` `  `// Driver code ` `public` `static` `void` `main(String[] args) ` `{ ` `     `  `    ``// Given string str  ` `    ``String str = ``"HASFJGHOGAKZZFEGA"``;  ` `     `  `    ``// Starting and Ending characters  ` `    ``char` `X = ``'A'``, Y = ``'Z'``;  ` `     `  `    ``// Function Call  ` `    ``longestSubstring(str, X, Y);  ` `} ` `} ` ` `  `// This code is contributed by divyeshrabadiya07 `

## Python3

 `# Python3 program for the above approach  ` ` `  `# Function returns length of longest  ` `# substring starting with X and  ` `# ending with Y  ` `def` `longestSubstring(``str``, X, Y):  ` `     `  `    ``# Length of string  ` `    ``N ``=` `len``(``str``) ` `     `  `    ``start ``=` `0` `    ``end ``=` `N ``-` `1` `    ``xPos ``=` `0` `    ``yPos ``=` `0` ` `  `    ``# Find the length of the string  ` `    ``# starting with X from the beginning  ` `    ``while` `(``True``):  ` `        ``if` `(``str``[start] ``=``=` `X):  ` `            ``xPos ``=` `start  ` `            ``break` `             `  `        ``start ``+``=` `1` ` `  `    ``# Find the length of the string  ` `    ``# ending with Y from the end  ` `    ``while` `(``True``):  ` `        ``if` `(``str``[end] ``=``=` `Y):  ` `            ``yPos ``=` `end  ` `            ``break` `         `  `        ``end ``-``=` `1` ` `  `    ``# Longest substring  ` `    ``length ``=` `(yPos ``-` `xPos) ``+` `1` ` `  `    ``# Print the length  ` `    ``print``(length) ` ` `  `# Driver Code  ` `if` `__name__ ``=``=` `"__main__"``: ` `     `  `    ``# Given string str  ` `    ``str` `=` `"HASFJGHOGAKZZFEGA"` ` `  `    ``# Starting and Ending characters  ` `    ``X ``=` `'A'` `    ``Y ``=` `'Z'` ` `  `    ``# Function Call  ` `    ``longestSubstring(``str``, X, Y)  ` ` `  `# This code is contributed by sanjoy_62 `

## C#

 `// C# program for the above approach   ` `using` `System; ` ` `  `class` `GFG{ ` `     `  `// Function returns length of longest  ` `// substring starting with X and  ` `// ending with Y  ` `static` `void` `longestSubstring(``string` `str,  ` `                             ``char` `X, ``char` `Y)  ` `{  ` `     `  `    ``// Length of string  ` `    ``int` `N = str.Length;  ` `    ``int` `start = 0;  ` `    ``int` `end = N - 1;  ` `    ``int` `xPos = 0;  ` `    ``int` `yPos = 0;  ` `     `  `    ``// Find the length of the string  ` `    ``// starting with X from the beginning  ` `    ``while` `(``true``) ` `    ``{  ` `        ``if` `(str[start] == X) ` `        ``{  ` `            ``xPos = start;  ` `            ``break``;  ` `        ``}  ` `        ``start++;  ` `    ``}  ` `     `  `    ``// Find the length of the string  ` `    ``// ending with Y from the end  ` `    ``while` `(``true``) ` `    ``{  ` `        ``if` `(str[end] == Y) ` `        ``{  ` `            ``yPos = end;  ` `            ``break``;  ` `        ``}  ` `        ``end--;  ` `    ``}  ` `     `  `    ``// Longest substring  ` `    ``int` `length = (yPos - xPos) + 1;  ` `     `  `    ``// Print the length  ` `    ``Console.Write(length); ` `}  ` ` `  `// Driver code ` `public` `static` `void` `Main() ` `{ ` `     `  `    ``// Given string str  ` `    ``string` `str = ``"HASFJGHOGAKZZFEGA"``;  ` `     `  `    ``// Starting and Ending characters  ` `    ``char` `X = ``'A'``, Y = ``'Z'``;  ` `     `  `    ``// Function call  ` `    ``longestSubstring(str, X, Y);  ` `} ` `} ` ` `  `// This code is contributed by sanjoy_62 `

Output:

```12
```

Time Complexity: O(N)
Auxillary Space: O(1) My Personal Notes arrow_drop_up Check out this Author's contributed articles.

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