# Length of the longest valid substring

Given a string consisting of opening and closing parenthesis, find length of the longest valid parenthesis substring.

Examples:

```Input : ((()
Output : 2
Explanation : ()

Input: )()())
Output : 4
Explanation: ()()

Input:  ()(()))))
Output: 6
Explanation:  ()(())
```

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

A Simple Approach is to find all the substrings of given string. For every string, check if it is a valid string or not. If valid and length is more than maximum length so far, then update maximum length. We can check whether a substring is valid or not in linear time using a stack (See this for details). Time complexity of this solution is O(n2.

An Efficient Solution can solve this problem in O(n) time. The idea is to store indexes of previous starting brackets in a stack. The first element of stack is a special element that provides index before beginning of valid substring (base for next valid string).

```
1) Create an empty stack and push -1 to it. The first element
of stack is used to provide base for next valid string.

2) Initialize result as 0.

3) If the character is '(' i.e. str[i] == '('), push index
'i' to the stack.

2) Else (if the character is ')')
a) Pop an item from stack (Most of the time an opening bracket)
b) If stack is not empty, then find length of current valid
substring by taking difference between current index and
top of the stack. If current length is more than result,
then update the result.
c) If stack is empty, push current index as base for next
valid substring.

3) Return result.```

Below is the implementations of above algorithm.

## C++

 `// C++ program to find length of the longest valid ` `// substring ` `#include ` `using` `namespace` `std; ` ` `  `int` `findMaxLen(string str) ` `{ ` `    ``int` `n = str.length(); ` ` `  `    ``// Create a stack and push -1 as initial index to it. ` `    ``stack<``int``> stk; ` `    ``stk.push(-1); ` ` `  `    ``// Initialize result ` `    ``int` `result = 0; ` ` `  `    ``// Traverse all characters of given string ` `    ``for` `(``int` `i=0; i

## Java

 `// Java program to find length of the longest valid ` `// substring ` ` `  `import` `java.util.Stack; ` `     `  `class` `Test ` `{ ` `    ``// method to get length of the longest valid ` `    ``static` `int` `findMaxLen(String str) ` `    ``{ ` `        ``int` `n = str.length(); ` `      `  `        ``// Create a stack and push -1 as initial index to it. ` `        ``Stack stk = ``new` `Stack<>(); ` `        ``stk.push(-``1``); ` `      `  `        ``// Initialize result ` `        ``int` `result = ``0``; ` `      `  `        ``// Traverse all characters of given string ` `        ``for` `(``int` `i=``0``; i

## Python

 `# Python program to find length of the longest valid ` `# substring ` ` `  `def` `findMaxLen(string): ` `    ``n ``=` `len``(string) ` ` `  `    ``# Create a stack and push -1 as initial index to it. ` `    ``stk ``=` `[] ` `    ``stk.append(``-``1``) ` ` `  `    ``# Initialize result ` `    ``result ``=` `0` ` `  `    ``# Traverse all characters of given string ` `    ``for` `i ``in` `xrange``(n): ` `     `  `        ``# If opening bracket, push index of it ` `        ``if` `string[i] ``=``=` `'('``: ` `            ``stk.append(i) ` ` `  `        ``else``:    ``# If closing bracket, i.e., str[i] = ')' ` `     `  `            ``# Pop the previous opening bracket's index ` `            ``stk.pop() ` `     `  `            ``# Check if this length formed with base of ` `            ``# current valid substring is more than max  ` `            ``# so far ` `            ``if` `len``(stk) !``=` `0``: ` `                ``result ``=` `max``(result, i ``-` `stk[``len``(stk)``-``1``]) ` ` `  `            ``# If stack is empty. push current index as  ` `            ``# base for next valid substring (if any) ` `            ``else``: ` `                ``stk.append(i) ` ` `  `    ``return` `result ` ` `  `# Driver program ` `string ``=` `"((()()"` `print` `findMaxLen(string) ` ` `  `string ``=` `"()(()))))"` `print` `findMaxLen(string) ` ` `  `# This code is contributed by Bhavya Jain `

## C#

 `// C# program to find length of  ` `// the longest valid substring ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG ` `{ ` `// method to get length of  ` `// the longest valid  ` `public` `static` `int` `findMaxLen(``string` `str) ` `{ ` `    ``int` `n = str.Length; ` ` `  `    ``// Create a stack and push -1 as  ` `    ``// initial index to it.  ` `    ``Stack<``int``> stk = ``new` `Stack<``int``>(); ` `    ``stk.Push(-1); ` ` `  `    ``// Initialize result  ` `    ``int` `result = 0; ` ` `  `    ``// Traverse all characters of ` `    ``// given string  ` `    ``for` `(``int` `i = 0; i < n; i++) ` `    ``{ ` `        ``// If opening bracket, push  ` `        ``// index of it  ` `        ``if` `(str[i] == ``'('``) ` `        ``{ ` `            ``stk.Push(i); ` `        ``} ` ` `  `        ``else` `// If closing bracket,  ` `             ``// i.e.,str[i] = ')' ` `        ``{ ` `            ``// Pop the previous opening  ` `            ``// bracket's index  ` `            ``stk.Pop(); ` ` `  `            ``// Check if this length formed  ` `            ``// with base of current valid  ` `            ``// substring is more than max  ` `            ``// so far  ` `            ``if` `(stk.Count > 0) ` `            ``{ ` `                ``result = Math.Max(result, i - stk.Peek()); ` `            ``} ` ` `  `            ``// If stack is empty. push current ` `            ``// index as base for next valid  ` `            ``// substring (if any)  ` `            ``else` `            ``{ ` `                ``stk.Push(i); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``return` `result; ` `} ` ` `  `// Driver Code  ` `public` `static` `void` `Main(``string``[] args) ` `{ ` `    ``string` `str = ``"((()()"``; ` `    ``Console.WriteLine(findMaxLen(str)); ` ` `  `    ``str = ``"()(()))))"``; ` `    ``Console.WriteLine(findMaxLen(str)); ` `} ` `} ` ` `  `// This code is contributed by Shrikant13 `

Output:

```4
6```

Explanation with example:

```Input: str = "(()()"

Initialize result as 0 and stack with one item -1.

For i = 0, str = '(', we push 0 in stack

For i = 1, str = '(', we push 1 in stack

For i = 2, str = ')', currently stack has [-1, 0, 1], we pop
from the stack and the stack now is [-1, 0] and length of current
valid substring becomes 2 (we get this 2 by subtracting stack top
from current index).
Since current length is more than current result, we update result.

For i = 3, str = '(', we push again, stack is [-1, 0, 3].

For i = 4, str = ')', we pop from the stack, stack becomes
[-1, 0] and length of current valid substring becomes 4 (we get
this 4 by subtracting stack top from current index).
Since current length is more than current result, we update result. ```

Another Efficient Approach can solve the problem in O(n) time. The idea is to maintain an array which stores the length of longest valid substring ending at that index. We iterate through the array and return the maximum value.

```
1) Create an array longest of length n (size of the input string) initialized to zero.
The array will store
the length of the longest valid substring ending at that index.

2) Initialize result as 0.

3) Iterate through the string from second character
a) If the character is '(' set longest[i]=0 as no valid sub-string will end with '('.
b) Else
i) if s[i-1] = '('
set longest[i] = longest[i-2] + 2
ii) else
set longest[i] = longest[i-1] + 2 + longest[i-longest[i-1]-2]

4) In each iteration update result as the maximum of result and longest[i]

5) Return result.```

Below is the implementations of above algorithm.

## C++

 `// C++ program to find length of the longest valid ` `// substring ` `#include ` `using` `namespace` `std; ` ` `  `int` `findMaxLen(string s) ` `{ ` `    ``if` `(s.length() <= 1) ``return` `0; ` `     `  `    ``//Initialize curMax to zero ` `    ``int` `curMax = 0; ` ` `  `    ``vector<``int``> longest(s.size(), 0); ` ` `  `    ``//Iterate over the string starting from second index ` `    ``for` `(``int` `i = 1; i < s.length(); i++) { ` `        ``if` `(s[i] == ``')'` `&& i - longest[i - 1] - 1 >= 0 && ` `            ``s[i - longest[i - 1] - 1] == ``'('``) { ` `                ``longest[i] = longest[i - 1] + 2 +  ` `                ``((i - longest[i - 1] - 2 >= 0)  ` `                ``? longest[i - longest[i - 1] - 2] : 0); ` `                    ``curMax = max(longest[i], curMax); ` `        ``} ` `    ``} ` `    ``return` `curMax; ` `} ` ` `  `// Driver program ` `int` `main() ` `{ ` `    ``string str = ``"((()()"``; ` `    ``cout << findMaxLen(str) << endl; ` ` `  `    ``str = ``"()(()))))"``; ` `    ``cout << findMaxLen(str) << endl ; ` ` `  `    ``return` `0; ` `} ` `// This code is contributed by Vipul Lohani `

## Python3

 `# Python3 program to find length of  ` `# the longest valid substring  ` ` `  `def` `findMaxLen(s): ` `    ``if` `(``len``(s) <``=` `1``): ` `        ``return` `0` `     `  `    ``# Initialize curMax to zero  ` `    ``curMax ``=` `0` ` `  `    ``longest ``=` `[``0``] ``*` `(``len``(s)) ` ` `  `    ``# Iterate over the string starting  ` `    ``# from second index ` `    ``for` `i ``in` `range``(``1``, ``len``(s)): ` `        ``if` `((s[i] ``=``=` `')'` `and`  `             ``i ``-` `longest[i ``-` `1``] ``-` `1` `>``=` `0` `and`  `             ``s[i ``-` `longest[i ``-` `1``] ``-` `1``] ``=``=` `'('``)):  ` `                ``longest[i] ``=` `longest[i ``-` `1``] ``+` `2` `                ``if` `(i ``-` `longest[i ``-` `1``] ``-` `2` `>``=` `0``): ` `                    ``longest[i] ``+``=` `(longest[i ``-`  `                                   ``longest[i ``-` `1``] ``-` `2``]) ` `                ``else``: ` `                    ``longest[i] ``+``=` `0` `                ``curMax ``=` `max``(longest[i], curMax) ` `    ``return` `curMax ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``Str` `=` `"((()()"` `    ``print``(findMaxLen(``Str``))  ` ` `  `    ``Str` `=` `"()(()))))"` `    ``print``(findMaxLen(``Str``)) ` ` `  `# This code is contributed by PranchalK `

Output:

```4
6```

Thanks to Gaurav Ahirwar and Ekta Goel for suggesting above approach.