solution: valid parentheses

Author: Jeongwon Na

valid-parentheses/njngwn.java

@@ -0,0 +1,24 @@
+// Time Complexity: O(n), n: s.length
+// Space Complexity: O(n), n: s.length (worst case: s="(((((((")
+class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> bracketStack = new Stack<>();
+
+        for (char ch : s.toCharArray()) {
+            if (ch == '(' || ch == '{' || ch == '[') { // open bracket
+                bracketStack.push(ch);
+            } else {    // close bracket
+                if (bracketStack.empty()) {
+                    return false;
+                }
+
+                char sp = bracketStack.pop();
+                if (!((sp == '(' && ch == ')') || (sp == '{' && ch == '}') || (sp == '[' && ch == ']'))) {
+                    return false;
+                }
+            }
+        }
+
+        return bracketStack.empty();
+    }
+}