Add Dart solution for Valid Parentheses

dart/valid_parentheses.dart

@@ -0,0 +1,28 @@
+/*
+Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
+
+An input string is valid if open brackets are closed by the same type of brackets and in the correct order.
+*/
+
+class Solution {
+  bool isValid(String s) {
+    final Map<String, String> pairs = {
+      ')': '(',
+      '}': '{',
+      ']': '[',
+    };
+    final List<String> stack = [];
+
+    for (int i = 0; i < s.length; i++) {
+      final String char = s[i];
+
+      if (pairs.containsValue(char)) {
+        stack.add(char);
+      } else if (stack.isEmpty || stack.removeLast() != pairs[char]) {
+        return false;
+      }
+    }
+
+    return stack.isEmpty;
+  }
+}