Added valid parentheses

LeeCode_Problems/20.Valid_Parentheses.cpp

@@ -0,0 +1,49 @@
+
+// Time Complexity: O(n);
+//Space Complexity: O(n) //auxiliary space
+#include<bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> stk;
+        int i=0;
+        for(int i=0;i<s.size();i++){
+            char ch=s[i];
+
+            if(ch=='(' || ch=='{' || ch=='['){
+                stk.push(ch);
+            }
+            else{
+                if(ch==')'){
+                    if(stk.empty() || stk.top()!='('){
+                        return false;
+                    }
+                    else{
+                        stk.pop();
+                    }
+                }
+                 else if(ch=='}'){
+                    if(stk.empty() || stk.top()!='{'){
+                        return false;
+                    }
+                    else{
+                        stk.pop();
+                    }
+                }
+                 else{
+                    if(stk.empty() || stk.top()!='['){
+                        return false;
+                    }
+                    else{
+                        stk.pop();
+                    }
+                }
+
+            }
+
+        }
+        return stk.empty();
+    }
+};