feat : valid-parentheses

Author: ekgns33

valid-parentheses/ekgns33.java

@@ -0,0 +1,54 @@
+/**
+ input : string s contains just (, ), {, }, [, ]
+ output : return if s is valid
+ valid means that parentheses are matched(balanced)
+ example
+ ((())) : valid
+ (((())) : invalid
+ {() : invalid
+
+ constraint :
+ 1) input string can be empty string?
+ nope. at least one character  >> if length is odd number return false
+ edge:
+ 1) if the length of string is odd number return false
+
+ solution 1)
+ ds : stack
+ algo : x
+ iterate through the string s
+ if opening bracket, add to stack
+ else check the top element of stack
+ if matched pop
+ else return false;
+ return false
+
+ tc : O(n)
+ sc : O(n) worst case : every character is opening bracket
+
+ */
+class Solution {
+  public boolean isValid(String s) {
+    //edge
+    if(s.length() % 2 == 1) return false;
+    // we can use deque instead
+    Stack<Character> stack = new Stack<>();
+
+    for(char c : s.toCharArray()) {
+      if(c == '(' || c == '{' || c == '[') {
+        stack.push(c);
+      } else {
+        if(stack.isEmpty()) return false;
+        if(c == ')') {
+          if(stack.peek() != '(') return false;
+        } else if (c == '}') {
+          if(stack.peek() != '{') return false;
+        } else if (c == ']'){
+          if(stack.peek() != '[') return false;
+        }
+        stack.pop();
+      }
+    }
+    return stack.isEmpty();
+  }
+}