20. Valid Parentheses

algorithms-and-data-structures/src/main/java/com/xtenzq/stack/Stacks.java

@@ -0,0 +1,39 @@
+package com.xtenzq.stack;
+
+import java.util.Map;
+import java.util.Stack;
+
+public class Stacks {
+
+    /**
+     * Determines if the input string is valid.
+     * <p>
+     * An input string is valid if:
+     * <ol>
+     *     <li>Open brackets must be closed by the same type of brackets;</li>
+     *     <li>Open brackets must be closed in the correct order;</li>
+     *     <li>Every close bracket has a corresponding open bracket of the same type.</li>
+     * </ol>
+     *
+     * @param s string to validate
+     * @return {@code true} if string is valid, {@code false} otherwise
+     * @implNote This method runs in {@code O(n)} time complexity, where {@code n} is the length of {@code s}.
+     * @see <a href="https://leetcode.com/problems/valid-parentheses/">20. Valid Parentheses</a>
+     */
+    public static boolean isValid(String s) {
+        Map<Character, Character> brackets = Map.of('(', ')', '{', '}', '[', ']');
+
+        Stack<Character> stack = new Stack<>();
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (brackets.containsKey(c)) {
+                stack.push(c);
+            } else {
+                if (stack.empty() || brackets.get(stack.pop()) != c) {
+                    return false;
+                }
+            }
+        }
+        return stack.empty();
+    }
+}

algorithms-and-data-structures/src/test/java/com/xtenzq/stack/StacksTest.java

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+package com.xtenzq.stack;
+
+import org.junit.jupiter.api.Test;
+
+import static com.xtenzq.stack.Stacks.isValid;
+import static org.junit.jupiter.api.Assertions.*;
+
+class StacksTest {
+
+    @Test
+    void isValid_case1() {
+        assertTrue(isValid("()"));
+    }
+
+    @Test
+    void isValid_case2() {
+        assertTrue(isValid("()[]{}"));
+    }
+
+    @Test
+    void isValid_case3() {
+        assertFalse(isValid("(]"));
+    }
+}