Create 20 Valid Parentheses

20 Valid Parentheses

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+20. Valid Parentheses
+Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
+
+An input string is valid if:
+
+Open brackets must be closed by the same type of brackets.
+Open brackets must be closed in the correct order.
+
+
+Example 1:
+
+Input: s = "()"
+Output: true
+Example 2:
+
+Input: s = "()[]{}"
+Output: true
+
+public boolean isValid(String s) {
+
+       Stack<Character> stack = new Stack(); //O(n)
+       for(char c :  s.toCharArray()){ //O(n)
+           if(c=='(' || c=='[' ||  c=='{'){
+               stack.add(c);
+           }else{
+               if(stack.isEmpty())return false;
+               if(c==']' &&  stack.peek()!='[')return false;
+               if(c==')' &&  stack.peek()!='(')return false;
+               if(c=='}' &&  stack.peek()!='{')return false;
+               stack.pop();
+           }
+       }
+
+       return stack.isEmpty();   
+    }