feat: add valid-parentheses solution in swift

leetcode/20 - validparenthesis/valid-parentheses.swift

@@ -0,0 +1,56 @@
+import Foundation
+
+/*
+ Constraints:
+
+ 1 <= string.length <= 104
+ string consists of parentheses only '()[]{}'.
+ */
+
+
+/**
+ Given a string containing just the characters `'('`, `')'`, `'{'`, `'}'`, `'['` and `']'`, determine if the input string is valid.
+
+ An input string is valid if:
+
+ 1. Open brackets must be closed by the same type of brackets.
+
+ 2. Open brackets must be closed in the correct order.
+
+ ~~~
+ // Example:
+ validateParentheses("()") // true
+ validateParentheses("()[]{}") // true
+ validateParentheses("(]") // false
+ validateParentheses("([)]") // false
+ validateParentheses("{[]}") // true
+ ~~~
+ */
+func validateParentheses(_ string: String) -> Bool {
+    var stack = ""
+
+    let closingParentheses = ")}]"
+    let openingParentheses: [Character : Character] = [
+        ")" : "(",
+        "}" : "{",
+        "]" : "["
+    ]
+
+    for char in string {
+        if closingParentheses.contains(char) {
+            // When it's a closing parenthesis:
+            // If last character is the matching opening parenthesis, pop last character
+            if stack.last == openingParentheses[char] {
+                stack.removeLast()
+            }
+            // It's invalid if:
+            // 1. stack is empty (stack.last == nil)
+            // 2. closing parenthesis is not a match to the opening parenthesis
+            else { return false }
+        }
+        else { stack.append(char) } // When it's an opening parenthesis, append the character
+    }
+
+    // If there is any character left in stack, it means that not all parentheses were closed
+    return stack.isEmpty
+}