add code of 20

Author: Li Li

Algorithms Basics/Chapter 4. Stack and Queue/20. Valid Parentheses.cs

@@ -0,0 +1,33 @@
+// condense and smart one, not a direct thought
+public class Solution {
+    public bool IsValid(string s) {
+        if (s == null || s.Length == 0) return false;
+        if (s.Length % 2 == 1) return false;
+        Stack<char> stack = new Stack<char>();
+        foreach (char c in s) {
+            if (c == '(') stack.Push(')');
+            else if (c == '[') stack.Push(']');
+            else if (c == '{') stack.Push('}');
+            else if (stack.Count == 0 || stack.Pop() != c) return false;
+        }
+        return stack.Count == 0;
+    }
+}
+
+// direct thought solution
+public class Solution {
+    public bool IsValid(string s) {
+        if (s == null || s.Length == 0) return false;
+        if (s.Length % 2 == 1) return false;
+        Stack<char> stack = new Stack<char>();
+        foreach (char c in s) {
+            if (c == '(' || c == '[' || c == '{') stack.Push(c);
+            else if (stack.Count == 0) return false;
+            else if (c == ')' && stack.Peek() == '(') stack.Pop();
+            else if (c == ']' && stack.Peek() == '[') stack.Pop();
+            else if (c == '}' && stack.Peek() == '{') stack.Pop();
+            else return false;
+        }
+        return stack.Count == 0;
+    }
+}