Merge pull request jitendrajat10099#20 from vrakr19/add-gfg-sol

add stack problem solution

Author: jitendrajat10099

GeeksforGeeks/Stack/longest_valid_parentheses.cpp

@@ -0,0 +1,32 @@
+/*
+    Problem Link: https://practice.geeksforgeeks.org/problems/longest-valid-parentheses/0
+*/
+
+#include<bits/stdc++.h>
+using namespace std;
+
+int main(){
+	int t;
+	cin>>t;
+	while(t--){
+		string str;
+		cin>>str;
+
+		stack<int> s;
+		s.push(-1);
+		int res=0;
+		for(int i=0;i<str.length();i++){
+			if(str[i]=='('){
+				s.push(i);
+			}else{
+				s.pop();
+				if(s.empty()){
+					s.push(i);
+				}else{
+					res = max(res,i-s.top());
+				}
+			}
+		}
+		cout<<res<<endl;
+	}
+}